Increase and abound - the idea is that their love be enlarged and made abundant, taken together giving the force of "increase to overflowing"! Clearly this was not a loveless church (eg they had a labor of love - 1Th 1:3-note), but they were not yet glorified (!) and therefore still had room to grow in love, the essential mark of the Christian faith as our Lord declared... 

And so it is not surprising that Paul prayed for increasing love for other churches such as the one at Philippi...

And this I pray, that your love may abound (perisseuo - superabound, overflow; present tense = as a continual supernatural outflow of your Spirit controlled and empowered life) still more and more (and here is the qualifier of such an overflowing love - it is not mindless sentimentality) in real knowledge and all discernment, so that you may approve the things that are excellent, in order to be sincere and blameless until the day of Christ (Php 1:9, 10-note)

In Paul's second letter we see his thanksgiving to God for having answered this prayer...

We ought always to give thanks to God for you, brethren, as is only fitting, because your faith is greatly enlarged, and the love of each one of you toward one another grows ever greater (2Thes 1:3)

Increase and abound in love - This is a prayer (in the aorist, optative) as discussed more fully below. Clearly the apostle Paul knew we depend on the Holy Spirit for the fruit of holiness such as love, and he expressed this dependence through prayer. Remember that prayer is not limited by time or place. Your prayers for your fellow believers (and your loved ones) will do more good than you realize, so stay devoted to prayer with an attitude of thanksgiving (Col 4:2-note)

Increase (4121)(pleonazo [word study] from pleion = more) means to cause to increase or to superabound. It suggests an abundance, an increase in number. It means to have or cause to have much, or more than enough. To have a surplus.

Pleonazo - 9x in 8v - Rom 5:20; 6:1; 2 Cor 4:15; 8:15; Phil 4:17; 1 Thess 3:12; 2 Thess 1:3; 2 Pet 1:8. NAS = cause*(1), grows...greater(1), have too much(1), increase(3), increased(1), increases(1), increasing(1), spreading(1).

Abound (4052)(perisseuo [word study] from perissós = abundant from peri = in sense of beyond) means to cause to overflow or superabound (quantitatively or qualitatively), to cause to excel. It means to exceed a fixed number or measure, to exist in superfluity.

Perisseuo - 39x in 35v - Matt 5:20; 13:12; 14:20; 15:37; 25:29; Mark 12:44; Luke 9:17; 12:15; 15:17; 21:4; John 6:12f; Acts 16:5; Rom 3:7; 5:15; 15:13; 1 Cor 8:8; 14:12; 15:58; 2 Cor 1:5; 3:9; 4:15; 8:2, 7; 9:8, 12; Eph 1:8; Phil 1:9, 26; 4:12, 18; Col 2:7; 1 Thess 3:12; 4:1, 10. NAS = abound(10), abounded(1), abounding(1), abundance(2), abundant(1), better(1), cause(1), cause*(1), excel(2), has an abundance(1), have an abundance(3), have more than enough(1), having abundance(1), increasing(1), lavished(1), left over(4), leftover(1), live in prosperity(1), make...abound(1), overflowed(1), overflowing(2), surpasses(1), surplus(2).

Hiebert writes that the two verbs increase and abound

are both aorist, optative of wish. They are virtually synonymous in import, and the use of both strengthens the expression of the prayer-wish. The former means "to become more, to increase, to be in abundance," while the latter means "to he present in abundance, to overflow" They may be rendered "to increase and to overflow" (Weymouth, Williams). The former may be viewed as "pointing to the process of growth" and the latter "to superlative attainment."' Then they may be thought of as standing in a relationship of cause and effect. The petition is not merely that their converts will "increase" but will be filled to overflowing in "love."

It is assumed that love is already present in their lives; the request is that it may increase to overflowing fullness. Its overflowing presence is the tangible evidence of a robust faith. Genuine Christian love, whose characteristics are set forth in 1 Corinthians 13, is the one thing in the Christian life that cannot be carried to excess. This abounding love will first of all express itself in their relations to "each other," the fellow-believers at Thessalonica. The reciprocal pronoun indicates that this love is to be shown mutually. But it must also be expressed as "for everyone else," not merely Christians in other places, but all men generally. For the persecuted Thessalonians this meant also loving their enemies, as Christ commanded (see note Matthew 5:44). To show love to their persecutors was the true safeguard against the natural tendency to retaliate when mistreated by outsiders. Such a love is not natural to man; it can he known and practiced only as it is received as a gift from the Lord and made to increase and abound by Him. (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996

Love (26) (agape [word study]) is unconditional, sacrificial love and a love that God is (1Jn 4:8,16) that God shows (Jn 3:16, 1Jn 4:9). Greek literature throws little light on this distinctive NT meaning. Agape love is the love of choice and of serving with humility. It is the highest kind of love, the noblest kind of devotion, the love of the will (intentional, conscious choice) and not done for appearance or motivated by emotional attraction. Agape is not based on  pleasant emotions or good feelings that might result from a physical attraction or a familial bond. From all of the descriptions of agape love, it is clear that true agape love is a sure mark of salvation. 

EBC comments that...

The goal of Paul's prayer for the Thessalonians is that the Lord will grant them "inner strength" to be "blameless" in holiness "in the presence of our God and Father" when the Lord Jesus returns. He looks forward to the time of final accounting. An overflow of love (1Th 3:12) is the only route to holy conduct in which no fault can be found (1Th 3:13). For unless love prevails, selfish motives inhibit ethical development by turning us toward ourselves and away from God and blameless living. The holiness that belongs to God is the ideal we must seek (cf. Lev 19:2; 1Pe 1:16-note). (Gaebelein, F, Editor: Expositor's Bible Commentary 6-Volume New Testament. Zondervan Publishing)

For one another and for all men - Paul had been thankful for their labor of love (1Th 1:3-note) and here is praying for an ever widening circle of love, like the Father has even for the unsaved (all men) and even their countrymen who had caused them suffering (1Th 2:14-note)

Just as we do for you - Paul daringly set himself as a standard of love to be emulated. We should live such Christian lives that we could tell young Christians, “Love other people just the way that I do.”

Someone has paraphrased this prayer as follows

The Lord enable you more and more to spend your lives in the interests of others, in order that He may so establish you in Christian character now, that you might be vindicated from every charge that might possibly be brought against you ...

R Larry Moyer has an note worth pondering regarding Paul's prayers for the saints (disciples)...

Paul’s emphasis on prayer for new converts is seen in other portions of his epistles such as 1Thessalonians 1:2, 3; 3:10, 11,12, 13; and Philippians 1:3, 4. What is striking is that Paul was keenly aware that no matter how many times he revisited the new believers, how many fellow workers he sent to labor among them, or how much communication he maintained with them, only the Holy Spirit could accomplish spiritual growth. God through His Spirit had to apply the truths of Scripture to their lives and cause them to be comprehended. Paul viewed his prayers to this end as an essential part of his follow-through.

Referring to Paul’s emphasis on prayer for new converts, Erdman notes,

A large portion of Paul’s time was spent in this pursuit. The “anxiety of all the churches” he had established found its natural expression in earnest intercession in their behalf. In the preface of his epistle written to any one of these churches, he states the requests which formed the substance of these daily supplications. There were in each case petitions appropriate to the particular need. However, these opening prayers of the apostle indicate exactly the requests which should be offered for the churches of the present day, and indeed for each individual Christian. They form possibly the most precious portions of the New Testament epistles. (from Charles R. Erdman, The Epistles of Paul to the Colossians and to Philemon Philadelphia: Westminster, 1966, 45)

One cannot help but wonder how much further along new converts would be if the amount of time given to criticism were directed to prayer for their spiritual growth. The Christian community has often had the practice and reputation of talking about prayer, but not praying enough. Moments given to persevering prayer could do more for new converts than hours of criticism or days of discussion on prayer. (Moyer, R. Larry: Assimilating New Converts into the Local Church: Bibliotheca Sacra: Volume 151, Issue 603, Page 340. Dallas TX: Dallas Theological Seminary)

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THE HEALTHIEST PLACE ON EARTH - IT'S one of the few places on earth where the air is as fresh and clean as it must have been millennia ago. Constant winds keep out pollution and germs, and the climate discourages the growth of native viruses.

It sounds like the healthiest place on earth. So why doesn't anyone want to live there? Because it's also the coldest place on earth. With temperatures that drop to minus 100 degrees Fahr­enheit, the South Pole is too frigid even for germs.

Some churches bear a striking resemblance to that sterile atmosphere. The truth of God is preached, Scriptures are metic­ulously quoted, and error has no chance to survive. But neither does life. The spiritual temperature is subzero, as evidenced by the cold shoulder given to the poor and needy (James 2:2, 3, 4, 5, 6). Those weak in the faith engage in icy arguments (Romans 14:1-note). Those who threaten to invade their comfortable cliques are left out in the cold (3John 5, 6, 7, 8, 9, 10). Unloved and unwelcomed, many people leave.

The church is to function as the body of Christ. As such, it should be warm, compassionate, and inviting. Our goal is not to keep out germs; it's to create an atmosphere where the spiritually sick can find healing. —M R De Haan II (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

SO THAT HE MAY ESTABLISH YOUR HEARTS UNBLAMABLE IN HOLINESS BEFORE OUR GOD AND FATHER AT THE COMING OF OUR LORD JESUS WITH ALL HIS SAINTS: eis to sterixai (AAN) humon tas kardias amemptous en hagiosune emprosthen tou theou kai patros emon en te parousia tou kuriou emon Iesou meta panton ton agion autou: (1Th 5:23; Romans 14:4; 16:25; 1Corinthians 1:8; Philippians 1:10; 2Thessalonians 2:16,17; 1Peter 5:10; 1John 3:20,21) (Ephesians 5:27; Colossians 1:22; 1John 3:20,21; Jude 1:24) (1Th 2:19; 4:15; 5:23; 1Corinthians 1:7; 15:23; 2Thessalonians 2:1) (Deuteronomy 33:2; Zechariah 14:5; 2Thessalonians 1:10; Jude 1:14)

So (1519) (eis) marks the conclusion of the petition and introduces a statement of its goal or aim ("contemplated result", Lenski). The verse thus reads not as another request but the goal  literally

to the end that he may strengthen your hearts blameless in holiness

The NIV dynamic paraphrase is somewhat inaccurate rendering this phrase as a separate prayer request...

May he strengthen your hearts so that you will be blameless and holy in the presence of our God and Father when our Lord Jesus comes with all his holy ones.

Hiebert commenting on establish your hearts notes that...

The Lord works through their increasing love to strengthen them—make them firm and solid (cf. 1Th 3:2-note)—in Christian character. Only as Christ develops in them the needed inner spiritual stability will they be able to stand firm and unmoved through whatever the future holds. Timothy had been sent to Thessalonica to help strengthen and encourage them (1Thes 3:2); this prayer is a reminder that however helpful the ministries of the Lord's servants may be, it is the Lord Himself who must work that inner stability in them. (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Establish (4741) (sterizo [word study] from histemi = to stand as in 1Pe 5:12-note "stand firm in" the true grace of God) means to make firm or solid, to set fast, to fix firmly in a place, to establish (make firm or stable), to cause to be inwardly firm or committed, to strengthen. The basic idea of the verb sterizo is that of stabilizing something by providing a support or buttress (a projecting structure of masonry or wood for supporting or giving stability to a wall or building), so that it will not totter.

Sterizo - 13x in 13v - Lk 9:51; 16:26; 22:32; Ro 1:11; 16:25; 1Th 3:2, 13; 2Th 2:17; 3:3; Jas 5:8; 1Pe 5:10; 2Pe 1:12; Re 3:2. NAS = confirm(1), determined(1), establish(2), established(2), fixed(1), strengthen(6), strengthening(1).

Barclay writes that sterizo 

Guzik has a wise remark noting that...

The heart must be made holy first. The devil wants us to develop a holy exterior while neglecting the interior, like whitewashed tombs, full of death (Matthew 23:27).

The following uses of sterizo demonstrate the various ways God uses to strengthen His saints and thus what and how Timothy was enabled to strengthen the faith of the saints in Thessalonica. These are the same "methods" believers today can utilize to strengthen the faith of their brethren, a need which is always present because every believer's faith is continually subject to testing. Have you ever been sent to strengthen another's faith? How would Timothy going to carry out Paul's charge to strengthen the faith of the saints at Thessalonica? Read on...

Believers
are strengthened...

By fervent, Scripturally based prayers of the saints...

May the Lord cause you to increase and abound in love for one another, and for all men, just as we also do for you that He may establish (sterizo) your hearts unblamable in holiness before our God and Father at the coming of our Lord Jesus with all His saints. (see notes 1Thes 3:12; 13)

Now may our Lord Jesus Christ Himself and God our Father, who has loved us and given us eternal comfort and good hope by grace, comfort and strengthen (sterizo) your hearts in every good work and word. (2Th 2:16, 17)

By looking and living for the Lord's return...

You too be patient  (makrothumeo - having a "long fuse" especially with difficult people - aorist imperative); strengthen (sterizo - aorist imperative Do it now - it's urgent!, active voice = you make the choice to do this) your hearts (the "control center" of your life), for the coming (parousia) of the Lord is at hand. (Jas 5:8)

By the God of all grace working through suffering...

And after you have suffered for a little while, the God of all grace, who called you to His eternal glory in Christ, will Himself perfect, confirm, strengthen and establish you. (1Pe 5:10-note)

By the truth of God's Word...

Therefore, I shall always be ready to remind you of these things, even though you already know them, and have been established (sterizo) in the truth which is present with you. (2Pe 1:12-note)

By the revealing of the mystery of the gospel...

Now to Him who is able to establish (sterizo) you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past (Ro 16:25-note);

By the strengthening ministered through His saints who come alongside...

(Peter) Simon, Simon, behold, Satan has demanded permission to sift you like wheat but I have prayed for you, that your faith may not fail; and you, when once you have turned again, strengthen (sterizo - aorist imperative conveys a sense of urgency. Do it effectively! When the opportunity presents itself, don't delay) your brothers. (Lk 22:31, 32)

(Timothy) and we sent Timothy, our brother and God's fellow worker in the gospel of Christ, to strengthen (sterizo) and encourage you as to your faith,

(Paul) For I long to see you in order that I may impart some spiritual gift to you, that you may be established (sterizo) (Ro 1:11-note).

In Scripture the "heart" (kardia) is a comprehensive term standing for the whole inner life, including thought, feeling, and will (cf. 1Th 2:4-note). Christian stability is not achieved through outward conformity to rules and regulations, needful as that is, but through the development of conscious inner strength and stability. Such a petition for the strengthening of the inner life of the believer is timely today when much attention is given to head and hand but the inner spiritual life is often neglected. (Ibid)

came to denote man’s entire mental and moral activities, and to stand figuratively for the hidden springs of the personal life, and so here signifies the seat of thought and feeling." (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson )

Heart (kardia). The heart is, first, the physical organ, the centre of the circulation of the blood. Hence, the seat and centre of physical life. In the former sense it does not occur in the New Testament. As denoting the vigor and sense of physical life, see Acts 14:17; Jas 5:5; Luke 21:34. It is used fifty-two times by Paul.

2. Of moral choice, 1Cor 7:37 2Cor 9:7.

It is equivalent to the inner man, Ep 3:16 (note); Ep 3:17 (note). Its characteristic is being hidden, Ro 2:28, 29 (note); Ro 8:27 (note); 1Cor 4:5; 14:25.

It is contrasted with the face, 1Th 2:17 (note); 2Cor 5:12; and with the mouth, Romans 10:8 (note).

Unblamable (273)(amemptos [word study] from a = negates following word + mémphomai = find fault) means irreproachable, faultless, without defect or blemish and thus describes not being able to find fault in someone or some thing (cf use in Heb 8:7 regarding the Old Covenant).  The idea is that the person is such that he or she is without the possibility of rightful charge being brought against them. Paul's desire for the Philippian saints is that there be no legitimate ground for accusation when the Lord returns to judge (see discussion of the bema or Judgment Seat of Christ for believers) (see below, 1Thes 3:13, for Paul's similar desire and prayer for the saints at Thessalonica).

Amemptos signifies that whatever charges might be made, no charge could be maintained. It points to the high standard God sets for His children (cf. 1John 2:1). It must ever be their aspiration and aim to so live that no fault may be found in them, that nothing in their conduct can be censured as evil. This adjective was  often used to characterize someone who is flawless in the sight of other people.

The related adverb  amemptos (differs by mark over the "o") is the very word archeologists have found on Christian tombs from ancient Thessalonica. When people wanted to identify a deceased friend or loved one as a Christian, they inscribed "amemptos" or "blameless" on his or her grave, such behavioral blamelessness (not just the imputed and forensic) is the Lord’s desire for His church.

Our position in Christ before God is blameless but Paul's prayer speaks of our daily walk and his desire for it to be blameless in holiness. He continues this theme in the next chapter exhorting them to excel still more

In holiness - Holiness is the sphere in which their spiritual career will unfold.

Holiness (42)(hagiosune from hagios = holy) basically refers to separation from what is common or unclean, and consecration to God (Lev 20:24-26 Acts 6:13; 21:28). Separation from the world involves more than keeping our distance from sinners but also means staying close to God. More than avoiding entertainment that leads to sin, it extends into how we spend our time and money. There is no way to separate ourselves totally from all sinful influences. Nevertheless, we are to resist the sin around us, without either giving up or giving in. When you know what God wants you to do, make a clean break with sinful practices. But beware of the trap of falling prey to keeping a list of do's and don't's or you will fall into the bondage of legalism.

Hagiosune refers to holiness not in the sense of describing the process of becoming "holy" but rather the quality of "being holy". It is the quality of holiness as an expression of the divine in contrast with the human. It has an ethical quality that reveals itself in purity of life. It indicates the state or condition of holiness.

Hiebert explains that...

The demand for holiness is rooted in the fact that by virtue of his acceptance of the atoning work of Christ, the believer has been separated from the world and set apart as belonging unto God. That which is devoted to God must he separated from sin. Genuine holiness is motivated by the obligations love imposes. (Ibid)

There are only 3 uses of hagiosune in the NT and here are the other two...

Romans 1:4 (note) who was declared the Son of God with power by the resurrection from the dead, according to the Spirit of holiness, Jesus Christ our Lord (Comment: Here hagiosune indicates a spirit or disposition of holiness which characterized Christ spiritually.)

2 Corinthians 7:1 Therefore, having these promises, beloved, let us cleanse ourselves from all defilement of flesh and spirit, perfecting holiness in the fear of God.

Hagiosúne and hagiótes (41) are qualities of character for which there need not be any proof; but for hagiasmos (38), the process of sanctification and the result of that process upon the individual, proof is necessary at each stage of its progressive achievement.

Hagios means to be set apart for a specific use or purpose. Includes the idea of taking something filthy, dirty and washing it and setting it apart as something brand new and useful for a different purpose--salvation

Hagiosune refers to a quality of life expressed in careful obedience to God.

Hagiosune refers to holiness of God pervading scheme of redemption and being made manifest in and by Christ. It emphasizes not an act of holiness but the state or condition of holiness. God's will is that our lives be characterized in every area by Christ-likeness in our attitudes and actions. The imminent "coming of our Lord Jesus" provides an apt incentive for all believers to pursue holiness.

Before our God and Father - This phrase removes this holiness from the realm of fallible human evaluation and lifts it into the presence of God Who is also our Father. The Christian must always be concerned about the evaluation that God our Father will make of our character. Our holiness of character and conduct are derived from Him and will be tested by Him. These grand attributes we are to prayerfully seek in this life will have their full realization at the coming of our Lord Jesus.

Hiebert comments that...

In keeping with the realization that the parousia of Jesus is not a single event, Walvoord accepts that

"this word coming here may not refer especially to the coming of Christ with His saints to earth, but rather specifically to the coming to heaven when they will be in the presence of the Father."

Then the reference is to the coming of Christ to catch up His people at the Rapture (see notes 1Thessalonians 4:13; 14; 15; 16; 17; 18), which marks the initiation of His personal presence with His redeemed people. The Judgment Seat of Christ (2 Cor 5:10 - see bema) is also the Judgment Seat of God (see note Romans 14:10), since the judgment of the saints takes place in the presence of God the Father on whose throne Christ is now seated (see note Revelation 3:21). Having been judged and rewarded, the saints will be presented to Christ as His spotless Bride. By virtue of their union with the Lord Jesus they will appear before God the Father as sons free from all fault and imperfection (see note Ephesians 5:27; cf. 1 John 3:2). (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

AT THE COMING OF OUR LORD JESUS WITH ALL HIS SAINTS: en te parousia tou kuriou emon Iesou meta panton ton agion autou: (1Thes 2:19; 4:15; 5:23; 1Corinthians 1:7; 15:23; 2Thessalonians 2:1) (Deuteronomy 33:2; Zechariah 14:5; 2Thessalonians 1:10; Jude 1:14)

At the coming - Remember that every chapter in this epistle ends with a reference to the return of Jesus Christ, and as in this section this great truth is applied to motivate expectant daily living. The imminent return of Jesus (1) is an evidence of salvation (1Th 1:9,10), (2) is a motivation for soul winning (1Th 2:17, 18, 19, 20), (3) is an encouragement for holy living (1Th 3:11, 12, 13),  (4) is a comfort in sorrow (1Th 4:18) (5) is a stimulus to have more confidence in the Lord (1Th 5:23, 24).

Coming (3952) (parousia [word study] from pareimi = to be present)  is a combination of two Greek words para = with, alongside +  ousia = being (ousia is the participial form of the verb eimi = to be) which together literally mean to be alongside.

Parousia - 24x in 24v - Matt 24:3, 27, 37, 39; 1 Cor 15:23; 16:17; 2 Cor 7:6f; 10:10; Phil 1:26; 2:12; 1 Thess 2:19; 3:13; 4:15; 5:23; 2 Thess 2:1, 8f; Jas 5:7f; 2 Pet 1:16; 3:4, 12; 1 John 2:28

Most lexicons in fact state that parousia is derived from pareimi (from para = near, with + eimi = to be) which means to be present, to be nearby, to have come.

Paul prayed for the Thessalonians that there would be no grounds of accusation because of unholiness. Compare a similar thought in these other NT passages...

1Cor 1:8 (awaiting eagerly the revelation of our Lord Jesus Christ) Who shall also confirm you to the end, blameless in the day of our Lord Jesus Christ.

2Cor 11:2 For I am jealous for you with a godly jealousy; for I betrothed you to one husband, that to Christ I might present you as a pure virgin.

Eph 5:25-27 Husbands, love your wives, just as Christ also loved the church and gave Himself up for her that He might sanctify her, having cleansed her by the washing of water with the word, that He might present to Himself the church in all her glory, having no spot or wrinkle or any such thing; but that she should be holy and blameless. (See notes Ephesians 5:25; 5:26; 5:27)

1Pet. 1:16-17 but like the Holy One who called you, be holy yourselves also in all your behavior because it is written, "YOU SHALL BE HOLY, FOR I AM HOLY." (See notes 1 Peter 1:16; 1:17)

Jude 24. Now to Him who is able to keep you from stumbling, and to make you stand in the presence of His glory blameless with great joy

Ryrie comments

Probably refers here to angels who will accompany the return of Christ (Mk 8:38), or possibly also holy men (cf. 1Th 4:14-note)

With all His saints - The expression  literally is all his holy ones indicates that when the Lord Jesus Christ returns, He will bring with Him all His holy ones, which I think without a doubt refers to believers but could also include angels (2 Thessalonians 1:7 "and to give relief to you who are afflicted and to us as well when the Lord Jesus shall be revealed from heaven with His mighty angels in flaming fire")

Morris basically agrees commenting that...

It is best to understand the ‘holy ones’ as all those bright beings who will make up His train be they angels or the saints who have gone before.

Vincent is somewhat dogmatic writing that...

Saints is often explained as angels; but the meaning is the holy and glorified people of God.  Hoi hagioi is uniformly used of these in NT and never of angels unless joined with aggeloiι. See Lk 9:26; Mk. 8:38; Acts 10:22. It is doubtful if hoi hagioi is used of angels in LXX. Zech. 14:5, which is confidently cited as an instance, is quoted at the conclusion of the Didache (16:7), clearly with the sense of glorified believers. Hagioi aggeloi appears Tob. 11:14; 12:15; Job 5:1. Angels has no connection with anything in this Epistle, but glorified believers is closely connected with the matter which was troubling the Thessalonians. See 1Thess 4:13. This does not exclude the attendance of angels on the Lord’s coming (see Mk. 8:38; Lk 9:26), but when Paul speaks of such attendance, as 2Th. 1:7, he says, with the angels (aggelon) of his power.

Hiebert sums up Paul's closing writing...

With this sublime prayer-wish the historical and personal portion of the epistle comes to a close. If Timothy's report had contained no account of "what is lacking in your faith" (1Thes 3:10) the letter might suitably have closed here. Rut the mention of the deficiencies in the faith of the readers prepares the way for the second half of the letter, which deals with the needed instructions and exhortations. (Ibid)

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A B Simpson writes that...