FOR THIS REASON, WHEN I COULD ENDURE IT NO LONGER, I ALSO SENT TO FIND OUT ABOUT YOUR FAITH: dia touto kago meketi stegon (PAPMSN) epempsa (1SAAI) eis to gnonai (AAN) ten pistin humon: (1Thes 3:2,6; Acts 15:36; 2Corinthians 7:5, 6, 7)

For this reason (dia touto) means in essence "since I knew that you were so liable to be persecuted, and since I feared that some might be turned from the truth by this opposition."

Endure (4722) (stego from steg = to cover, conceal, stege = roof) had a fairly broad range of meanings including to cover, to protect, to hold back, to hide,  to bear,  to endure or to persist.

Stego means to protect by covering or  to cover closely (so as to keep water out). Thus stego is found in secular Greek writings - "the camp protects men against the cold" (Plato); "a house protects men".

The related word steganos meant covering or sheltering (think about this as you study the use of stego in 1Corinthians 13:7 below).

Figuratively, stego derives it's meaning from the fact that by covering it keeps off something which threatens which then is taken to mean to bear up under.

At the core of its meaning stego denotes an activity or state which blocks entry from without or exit from within. Hence to protect by covering, as with a tight ship or roof.

Vine writes that stego...

signifies either that of which it is predicated supports what is placed upon it or covers what is placed underneath it. The former idea is prominent here and in 1Th 3:5-note  (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson or Logos)

Also sent (3992)(anapempo from aná =again + pémpo = send) means to send again or send back again

Find out (to get to know) (1097) (ginosko) refers to acquire information through some means, in this case learning through sense perception (hearing). It means to get to know or to ascertain. Paul desired to learn the condition of their faith, to gain experiential confirmation on the matter of how their faith was holding out under persecution.

Find out about your faith - In the present context their faith refers to their fidelity or steadfastness in the gospel.  Timothy's mission served a dual objective, to strengthen the faith of the converts and to obtain for Paul information concerning their welfare.

Richison makes a pithy point writing that...

No doubt all phonies will throw in the towel quickly. Persecution always sorts out the fakers.

The church popular is the church polluted.

The church persecuted is the church purified.

If the church receives nothing but favor from the world system then there is something wrong with the church. The world hates the church when it finds out what the church believes. (Ref)

If the world hates you, you know that it hated Me before it hated you. If you were of the world, the world would love its own. Yet because you are not of the world, but I chose you out of the world, therefore the world hates you (John 15:18, 19).

Faith (4102)(pistis) is synonymous with trust or belief and is the conviction of the truth of anything, but in Scripture usually speaks of belief  respecting man's relationship to God and divine things, generally with the included idea of trust and holy fervor born of faith and joined with it. Note that this discussion of pistis is only an overview and not a detailed treatise of this vitally important subject.

Wayne Grudem defines faith that saves one's soul...

FOR FEAR THAT THE TEMPTER MIGHT HAVE TEMPTED YOU, AND OUR LABOR SHOULD BE IN VAIN: me pos epeirasen (3SAAI) humas ho peirazon (PAPMSN) kai eis kenon genetai (2SAMS) ho kopos hemon: (Matthew 4:3; 1Corinthians 7:5; 2Corinthians 2:11; 11:2,3,13, 14, 15; Galatians 1:6, 7, 8, 9; Ephesians 4:14; James 1:13,14) (1Thes 2:1; Isaiah 49:4; Galatians 2:2; 4:11; Philippians 2:16)

For fear that (3381) (mepos from me = not, lest + pos = by any means) means lest by any means, that in no way, that by no means, lest perhaps. It serves as a marker of negative purpose, often with the implication of apprehension (fearful anticipation) as in the present context.

Wuest adds the phrase lest by any means - Barnes comments that Paul is referring to

Either by allurements to apostasy, set before you by your former heathen friends; or by the arts of false teachers; or by the severity of suffering. Satan has many methods of seducing men from the truth, and Paul was fearful that by some of his arts he might be successful there. (Albert Barnes. Barnes NT Commentary)

Regarding the power of the Tempter to tempt, Paul explains that he forgave the Corinthians

in order that no advantage be taken of us by Satan for we are not ignorant of his schemes. (2 Corinthians 2:11)

Comment: In this context Paul's point was that Satan can defeat a Christian or a congregation either through that Christian condoning sin or his refusal to forgive a repentant sinner.

The tempter (3985) (peirazo [word study] from the noun peira = test from peíro = perforate, pierce through to test durability of things) is a morally neutral word simply meaning “to test”. For example it may mean to put to the test and so to search out or to discover what kind of person someone is. And so it may he used of God or Christ in a good sense as putting men to the test that they may stand approved, as when Abraham was tested by God's request to him to offer Isaac, his son of promise, his only begotten son (He 11:17-note).

Peirazo on the other hand when used of the devil always has a bad connotation, meaning to test or tempt in order to disapprove. It is always the devil's aim to entice men to sin and bring them to a fall as in the temptation of Christ

Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil (Mt 4:1)

And so the meaning of peirazo depends on the intent of the one giving the test and also on the response of the one tested. (See study of another word for testing dokimazo)

In this passage the definite article (Greek = ho corresponds to English article "the") precedes the verb peirazo (ho peirazon), and is clearly a reference to specific tempting one, the Devil, Satan, the Serpent, the Evil one, Lucifer. Paul however does not now identify this nefarious foe by his name but by his characteristic activity. The present tense the indicating that temptation is his continual evil activity. It pictures him as persistently engaged in the effort to destroy the faith of the Thessalonians through temptation. He never gives up his sinister efforts. Do believers really understand and believe this today? Remember that Satan's tests are never with a good end in view but are always calculated to do us harm.

Satan is referred to as the Tempter only here and in Matthew 4...

And the tempter (peirazo) came and said to Him, "If You are the Son of God, command that these stones become bread." (Matthew 4:3)

Comment: He who was the tempter of our Lord is also the tempter of His people.

Tempted (3985) (peirazo) is the same verb discussed above but without the definite article preceding and in a different verb tense, aorist., referring here to a past fact. In other words Paul takes for granted that the saints have already been tempted by the tempter.

Vine comments that...

the point tense (aorist tense) indicates that the apostle feared not merely an assault on their faith, but a successful assault = “that the tempter had succeeded in tempting you.” Satan was doubly active, he had hindered the missionaries from continuing the work at Thessalonica; had he succeeded in overthrowing what they had done? (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson or Logos)

Richison warns us that...

The Devil will take advantage of your vulnerability. He knows about your history. He knows where he can stick it to you.

Temptation is an awful thing particularly when both Satan and your sin capacity join forces against you. At times, Satan will move into your family and create problems in your household. He hates harmonious Christian homes. He may, for example, attempt to invade the bedroom.

Do not deprive one another except with consent for a time, that you may give yourselves to fasting and prayer; and come together again so that Satan does not tempt you because of your lack of self–control (1Corinthians 7:5).

Satan loves misunderstanding, unkindness and thoughtlessness. He wants couples to take each other for granted. He wants them to misuse sex. He delights to see physical and emotional bonds broken and partners drift apart. The Devil may bring a third person onto the scene which can result in cheating, infidelity, and eventual divorce. (Ref)

Labor (2873) (kopos [word study]) is used in secular Greek of “a beating,” “weariness” (as though one had been beaten) and “exertion,” was the proper word for physical tiredness induced by work, exertion or heat. Kopos speaks of great effort and exertion, to the point of sweat and exhaustion. It pictures one who is physically become worn out, weary or faint. It describes intense toil even to the point of utter exhaustion if necessary.  The work described by kopos  left one so weary that it was as if the person had taken a beating. Kopos speaks not so much of the actual exertion as the weariness which follows the straining of all one's powers to the utmost.

Paul had used kopos earlier in the letter writing that...

you recall, brethren, our labor (kopos) and hardship, how working night and day so as not to be a burden to any of you, we proclaimed to you the gospel of God. (1Th 2:9-note) (Comment: They worked and ministered, either one of which is enough to sap one's energy, thus we can understand Paul's allusion again to labor, although of course he is not concerned about his physical labor being in vain but his spiritual labor.)

Labor in vain - As would be the case if the Thessalonians were turned away from the faith by tempter. Paul remembered the response the Thessalonians had given to the preaching of the gospel for three Sabbaths in the synagogue (Acts 17:1, 2, 3, 4). As some commentators suggest Paul had been driven out before it could be established whether the Thessalonians had expressed merely an emotional reaction (which would have made the labor in vain) or had truly attained to genuine faith and conversion. This is always a good principle to keep in mind - we must assiduously follow up on those who have recently professed faith in the Gospel of Christ, lest they be shown to be only professors and not possessors of eternal life.

Hiebert comments that Paul...

knew that the tempter had been busy at Thessalonica. But at the time of sending he did not know the outcome of that tempting activity. This uncertainty is indicated by his use of the aorist subjunctive in the expression "and our efforts might have been useless." It was his apprehension of this dread possibility that caused Paul such anxiety about the Thessalonians. There was the possible outcome that "our efforts," the wearisome toil the missionaries had expended at Thessalonica, might turn out to "have been useless" ("should come to nothing"—Darby). The possibility was that it should prove to be an empty thing, like a nut without a kernel. Paul was fully aware that the final outcome of his labors was dependent upon the steadfastness of his converts. The return of Timothy with his report brought the glad news that Paul's fears had not been realized. But that does not mean that his fears had been unwarranted. Knowing the fierceness of the enemy attack and the fickleness of human nature, Paul's fears were altogether reasonable. (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Writing to the saints at Corinth Paul said...

But I am afraid, lest as the serpent deceived Eve by his craftiness, your minds should be led astray from the simplicity and purity of devotion to Christ (Paul feared that they would abandon their simple devotion to Christ in favor of the sophisticated error of the false apostles, just as Eve did with the chief falsifier!). 4 For if one comes (false apostles came into the Corinthian church from the outside just as Satan did into the Garden) and preaches another Jesus whom we have not preached, or you receive a different spirit which you have not received, or a different gospel which you have not accepted, you bear this beautifully (Paul is not endorsing their acceptance of heresy but is speaking with intense irony, in essence chiding them for their spiritual gullibility and lack of discernment!)...13 For such men are false apostles, deceitful workers, disguising themselves as apostles of Christ. 14 And no wonder, for even Satan disguises himself as an angel of light. 15 Therefore it is not surprising if his servants also disguise themselves as servants of righteousness; whose end shall be according to their deeds.  (2Corinthians 11:3,4,13, 14, 15)

In Galatians Paul voices similar concerns writing...

And it was because of a revelation that I went up; and I submitted to them the gospel which I preach among the Gentiles, but I did so in private to those who were of reputation, for fear that I might be running, or had run, in vain (kenos). (Galatians 2:2)

MacArthur explains in vain here writing that "Paul hoped the Jerusalem leaders would support his ministry to the Gentiles and not soften their opposition to legalism. He did not want to see his ministry efforts wasted because of conflict with the other apostles {MacArthur Study Bible: Word}

I fear for you, that perhaps I have labored (kopiao = intense toil which an include straining to point of exhaustion) over you in vain (eike - for nothing - it means without any result or for no purpose) (Gal 4:11)

Comment: His intense efforts of evangelizing and discipling the Galatians churches might prove futile if they fell back into legalism. Paul would write this same concern to professing Christians today who seek to attain holiness by legal observances!

KJV Bible Commentary sounds a warning note "Paul’s labor would have been in vain if all that was accomplished was for the Galatians to exchange their pagan religion for the old abrogated legalism of the Jews. Turning to legalism is equivalent to rejecting the gospel and renouncing Christ. Paul’s admonition to the Galatians should be a solemn warning to Christians not to sacrifice their spiritual liberty in Christ for the slavery of forms and ceremonies." { KJV Bible Commentary: Nelson)

Finally writing to his beloved saints at Philippi Paul exhorts them to be lights in a midst of the crooked, perverted, pagan, dark and depraved world around them...

holding fast (present tense = continually holding on to in sense of holding it tight or holding forth in sense of holding out as if offering) the word of life (this phrase is first in the Greek sentence for emphasis and refers to the Gospel, which is life and which gives life - it is a supernatural word which has life in itself and which leads to real life in Christ - cp Col 1:6-note{referring to the gospel in Col 1:5-note}, 1Th 2:13-note), so that in the day of Christ I may have cause to glory because I did not run in vain (kenos) nor toil in vain (kenos). (Php 2:16-note)

Vain (2756) (kenos) means literally to be without something material and thus means empty or without content. It was used with this literal meaning (as in Mk 12:3 "they took him and beat him and sent him away empty handed"). More often kenos is used figuratively referring to things that lack effectiveness and thus are futile, useless, of no purpose or without result. Kenos is used to refer to endeavors, labors, acts, which result in nothing and thus are vain, fruitless, without effect and will not succeed. Kenos can refer to being devoid of intellectual, moral, or spiritual value.

Guzik comments that...

In the parable of the soils (Matthew 13:1-23) Jesus described the seed that withered under the heat of trials. If the Thessalonians withered, Paul’s hard work as a farmer among them would have born no harvest. Paul did something to help prevent the Thessalonians from falling under their affliction. He sent Timothy to them, because those who are in affliction need the help of other godly people.

TDNT says kenos...

1. Literally the meaning is “empty” usually things, but also persons.

2. Figuratively the reference is to vain or frivolous persons or futile things, e.g., opinions, boastings, speech, and cf. the expression eis kenon, “in vain.” (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

Trench writes that when used

of persons, kenos predicates not merely an absence and emptiness of good, but, since the moral nature of man endures no vacuum, the presence of evil. It is thus employed only once in the NT, namely at Ja 2:20 where the (anthropos kenos)  is one in whom the higher wisdom has found no entrance, but who is puffed up with a vain conceit of his own spiritual insight, ‘aufgeblasen,’ as Luther has it.

NIDNTT notes that in Classic Greek kenos is...

found frequently from Homer onwards (and) means empty, as opposed to pleres (full). It is thus used mostly lit. of things (e.g. an empty jug, pit [cf. Ge 37:24], or house). But it is also occasionally used of persons (e.g. with empty hands [cf. Ge 31:42]).

When used metaphorically in connection with things, kenos means either lacking content (e.g. kenoi logoi, empty words [cf. Plato, Lach., 196b]), or a missing effect (especially in the expression eis kenon, in vain [The Flinders Petrie Papyri, II, 37, 1b, 12; Josephus, Ant., 19, 96]). With people, kenos means hollow, shallow, lacking in judgment (tou nou kenos, empty in mind [Soph., OC, 931]), and also in the ethical sense of ineffectual, vain (Soph., Ant., 709).

(In the LXX of the OT) kenos is used mostly in the lit. sense of empty (e.g. Jer. 14:3, vessels; Ex. 3:21 and Deut. 15:13, empty-handed). But it also has a distinctive metaphorical sense. Judges 9:4 and Judges 11:3 speak of andres kenoi, worthless men, who are not counted among the people of Yahweh and who are ready and willing to perform any kind of deed, even murder. The prophets spoke of turning away from Yahweh as giving oneself to vanity. The help that Israel sought from the Egyptians was worthless and empty (Isa. 30:7). Israel has forgotten Yahweh and offers sacrifice to vanity or nothing, i.e. the idols (Jer 18:15). The idea of emptiness was sharpened in the message of the prophets as a deceptive power upon which no reliance can be placed (Isa. 29:8). It is unmasked as senselessness. Only Yahweh can help. The Lord’s chosen ones shall not toil in vain (Isa. 65:23).

The term is found most frequently in the cries of Job. He resents the vain words (Job 27:12) and the empty comfort (Job 21:34) of his friends. He sees not only the things around him but his own life sink into nothing. He laments the months of emptiness (7:3) and vain hope (7:6). He cries out: “Leave me alone! For my life is nothing” (7:16). He sees himself delivered to the curse of nothingness without reason. Unless Yahweh rescues him, he can only perish. (Brown, Colin, Editor. New International Dictionary of NT Theology. 1986. Zondervan) 

Kenos is used 60 times in the Septuagint (LXX) (Gen. 31:42; 37:24; Ex 3:21; 5:9; 23:15; 34:20; Lev. 26:16, 20; Deut. 15:13; 16:16; 32:47; Jdg. 7:16; 9:4; 11:3; Ruth 1:21; 3:17; 1 Sam. 6:3; 2 Sam. 1:22; 2 Ki. 4:3; Neh. 5:13; Job 2:3, 9; 6:5f; 7:3, 6, 16; 9:17; 15:31, 35; 20:18; 21:34; 22:6, 9; 27:12; 31:34; 33:21; 34:20; 39:16; Ps. 2:1; 7:4; 25:3; 31:6; 107:9; Prov. 23:29; Isa. 29:8; 30:7; 32:6; 45:18; 59:4; 65:23; Jer. 6:29; 14:3; 18:15; 27:9; 46:11; 51:58; Hos. 12:1; Mic. 1:14; Hab. 2:3). Here are a few representative uses of kenos in the LXX...

Exodus 3:21 "And I will grant this people favor in the sight of the Egyptians; and it shall be that when you go, you will not go empty-handed (Lxx = kenos).'"

Deuteronomy 32:47 "For it (Word of God) is not an idle (Lxx = vain) word for you; indeed it is your life. And by this word you shall prolong your days in the land, which you are about to cross the Jordan to possess."

Judges 9:4 And they gave him seventy pieces of silver from the house of Baal-berith with which Abimelech hired worthless (Lxx = kenos) and reckless fellows, and they followed him.

Ruth 1:21 "I went out full, but the LORD has brought me back empty (Lxx = kenos). Why do you call me Naomi, since the LORD has witnessed against me and the Almighty has afflicted me?"

Ruth 3:17 And she said, "These six measures of barley he gave to me, for he said, 'Do not go to your mother-in-law empty-handed (Lxx = kenos).'"

Psalm 2:1 Why are the nations in an uproar, And the peoples devising a vain (Lxx = kenos) thing?

Kenos is used 18 times in 16 verses in the NT...

Mark 12:3 "And they took him, and beat him, and sent him away empty-handed. (The parable of the vineyard in which the tenants send back the master's servants empty-handed, cf Luke 20:10)

Luke 1:53 "He has filled the hungry with good things; And sent away the rich empty-handed.

Luke 20:10 "And at the harvest time he sent a slave to the vine-growers, in order that they might give him some of the produce of the vineyard; but the vine-growers beat him and sent him away empty-handed. 11 "And he proceeded to send another slave; and they beat him also and treated him shamefully, and sent him away empty-handed.

Acts 4:25 who by the Holy Spirit, through the mouth of our father David Thy servant, didst say, 'Why did the Gentiles rage, And the peoples devise futile things (things that will not succeed)?

1 Corinthians 15:10 But by the grace of God I am what I am, and His grace toward me did not prove vain (empty, lacking in anything which might or should be possessed); but I labored even more than all of them, yet not I, but the grace of God with me...14 and if Christ has not been raised, then our preaching is vain, (here kenos = devoid of any spiritual value) your faith also is vain...58 Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your toil is not in vain in the Lord (vain here = labor which yields no return).

2 Corinthians 6:1 And working together with Him, we also urge you not to receive the grace of God in vain--

Galatians 2:2 And it was because of a revelation that I went up; and I submitted to them the gospel which I preach among the Gentiles, but I did so in private to those who were of reputation, for fear that I might be running, or had run, in vain.

Ephesians 5:6 (note) Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. (Comment: The words were empty, hollow, without the substance of truth or reality. They sounded plausible, but were devoid of truth. and "employed to palliate heathen vices” {Vincent}. Expositors Greek Testament notes that empty words is a general expression "applying to all who sought by their sophistries to palliate the vices in question or make them appear to be no vices”)

Philippians 2:16 (note) holding fast the word of life, so that in the day of Christ I may have cause to glory because I did not run in vain nor toil in vain.

Colossians 2:8 (note) See to it that no one takes you captive through philosophy and empty deception (Deceit is always an empty thing. Only what is true is solid), according to the tradition of men, according to the elementary principles of the world, rather than according to Christ.

1Thessalonians 2:1 (note) For you yourselves know, brethren, that our coming to you was not in vain,

1Thessalonians 3:5 (note) For this reason, when I could endure it no longer, I also sent to find out about your faith, for fear that the tempter might have tempted you, and our labor should be in vain. (a "failure")

James 2:20 But are you willing to recognize, you foolish (kenos) fellow, that faith without works is useless (nekros = dead)? (Comment: Vine has this note -  In James 2:20 the vain man {kenos} is one empty of divinely imparted wisdom; in James 2:26 the vain religion {mataios} is that which produces nothing profitable; kenos stresses lack of quality, mataios lack of effect).

Find out (1097)(ginosko) has the basic meaning of taking in knowledge in regard to something or someone, knowledge that goes beyond the merely factual. By extension, the term frequently was used of a special relationship between the person who knows and the object of the knowledge. It was often used of the intimate relationship between husband and wife and between God and His people.

What was Paul concerned about? Whether they were temporary or had firmly taken root. In His parable of the soils Jesus explains that...