For (1063)(gar) is a conjunction basically introducing an explanation and here extends and develops Paul's point in 1Thessalonians 2:4 (note) about speaking not to please men but God. Paul explains that (1) they avoided flattery, (2) they were not seeking to promote their own financial gain and (3) they were not looking for fame, honor or praise. In fact, so concerned was Paul to avoid any hint of financial self-interest that could compromise the gospel that he took an additional precaution. Dear preacher, teacher, church leader, why do you do what you do in the church of Jesus Christ? The world sees what we do but God sees why we do it!

The NIV Application Commentary observes that...

Behind this concern over means and motives is Paul’s obvious concern for the integrity of the message. He deliberately avoided behavior or actions that might lead people to doubt or suspect the integrity of the message or the sincerity of his preaching. As an apostle he had a right to be supported in his ministry (1Cor 9:3-18). But for the sake of the integrity of the message and for the sake of the Thessalonians, he voluntarily gave up that right.

Never - Literally "never (oute) at some time (pote)".

Never (3777) (oute from ou = not in an absolute sense + te = even) means Neither, nor, not even, and not, also not. The missionaries never pandered to their feelings nor did they soften the demands of the gospel. They  never stooped to flattery or insincerity in order to achieve results but remained honest and transparent, and free of hypocrisy.

Some time (4218) (pote from poú = as adv. of place, somewhere, nearly: about, a certain place + te = ever) at some time, ever. When, whenever. At some time, one time or another, once used both of time past and future.

At no time did the Paul and his team use manipulation to achieve God's purposes, as Paul explained in other passages...

For such men (those who cause dissensions and hindrances contrary to the teaching which you learned) are slaves, not of our Lord Christ but of their own appetites; and by their smooth and flattering speech they deceive the hearts of the unsuspecting. (see note Romans 16:18).

For we are not, as so many, peddling the word of God but as of sincerity, but as from God, we speak in the sight of God in Christ (2Cor 2:17) (Comment: Even in Paul's day, there were many false teachers who were corrupting the sincere teaching of God's Word and who peddled the Word for profit.)

They never used flattery, of which Solomon writes...

a flattering mouth works ruin. (Pr 26:28b)

He who rebukes a man (cf note 2 Ti 4:2) will afterward find more favor than he who flatters with the tongue. (Pr 28:23)

Flattering (2850) (kolakeia from kolax = a fawner) refers to cajolery which is an attempt to persuade by use of insincere speech or exaggerated praise. Kolakeia was a well-known secular Greek term for a practice despised as much as "boldness" was valued, and which is occasionally connected with financial gain.  Flattery was a well-known, much despised practice in Paul's day.

Kolakeia contains the idea of deception for selfish ends. It is flattery not merely for the sake of giving pleasure to others but for the sake of self interest. It is deception by "slick" eloquence (sounds like many politicians we know doesn't it?) with the idea of winning over the listener's heart in order to exploit not edify.

Hiebert writes that...

Flattery does not simply mean complimentary words intended to tickle the ears of the hearers pleasurably. It is rather the smooth-tongued discourse of the orator aimed at making a favorable impression that would gain influence over others for selfish advantage...Paul denies that he and his fellow workers had used the preaching of the gospel as a foil for securing selfish advantage. That such a charge could easily he given a ring of plausibility is clear from the known conduct of the heathen rhetoricians of the day. Nor was the early church exempt from this evil. Barclay reminds us that the early Christian churches were plagued with "people who did attempt to cash in on their Christianity" (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Lightfoot writes that...

Kolakeia, a word which occurs here only in the New Testament, is defined both by Theophrastus ... and Aristotle ... to involve the idea of selfish motives. It is flattery not merely for the sake of giving pleasure to others but for the sake of self-interest"

Moulton and Milligan write that kolakeia...

carries with it the idea of the tortuous methods by which one man seeks to gain influence over another, generally for his own ends, and when we keep in view the selfish conduct of too many of the heathen rhetoricians of the day...we can easily understand how such a charge might come to be laid against the Apostles. (Moulton, J. H., & Milligan, G.. The Vocabulary of the Greek Testament.  London: Hodder and Stoughton. 1930).

ISBE writes that...

The term “flattery” as used by the RSV is always associated with deception.

Green writes that...

The ancients recognized that flattery was evidence of the bad character of the person who used it to persuade or move others to action. It was a way to gain the favor of someone, equal to the way one could gain another’s favor through giving gifts or entertaining the person one wished to influence. Plato advised that one should shun any kind of flatterer, while sometime later Plutarch stated that a flatterer corrupts the morals of the young and only pretends to be a friend. Dio Chrysostom warned that flattery was a characteristic of charlatans and sophists. Plutarch was so concerned about flatterers that he wrote a whole treatise about how to distinguish between them and true friends. As communicators of the truth of God and as those who sincerely cared for the well-being of their hearers, the apostles never flattered their audience in an attempt to gain something for themselves. The Thessalonians knew this well. (Green, G. L. The letters to the Thessalonians. The Pillar New Testament Commentary. Grand Rapids, Mich.; Leicester, England: W.B. Eerdmans Pub.; Apollos)

Richison writes that...

Flattery operates under ulterior motives. Flattery misleads people by making them think you believe in them more than you do. This is the sin of smooth words. Putting people off–guard by smooth words is lying. (2:5; 2:5b)

The flatterer compliments you merely as a ploy to win your favor  or to gain power over you. A flatterer is a man that tells you your opinion and not his own. On the other hand, the smaller we become, the more room God has to work.

Wiersbe writes that...

a flatterer is a person who manipulates rather than communicates. A flatterer can use either truth or lies to achieve his unholy purpose, which is to control your decisions for his own profit. Some people even flatter themselves. “For he flatters himself in his own eyes” (Ps. 36:2). This was the sin of Haman, that evil man in the Book of Esther. He was so interested in flattering himself that he even plotted to slaughter all the Jews to achieve that goal. Some people try to flatter God. “Nevertheless they [Israel] did flatter Him [God] with their mouth, and they lied unto Him with their tongues” (Ps. 78:36). Flattery is another form of lying. It means saying one thing to God with our lips while our hearts are far from Him (Mark 7:6). Some Christians try to win friends and influence people by appealing to their egos. A true ministry of the Gospel deals honestly (but lovingly) with sin and judgment and leaves the unbeliever with nothing to boast of in himself. (Wiersbe, W: Bible Exposition Commentary. 1989. Victor or Logos)

Flatterers look like friends just as wolves resemble dogs. Treachery lurks in honeyed words — Danish proverb

Aristotle distinguished between friends and flatterers writing

The man who always joins in the pleasures of his companions, if he sets out to be pleasant for no ulterior motive, is Obsequious (marked by or exhibiting a fawning or excessive attentiveness); if he does so for the sake of getting something by it in the shape of money or money’s worth, he is a Flatterer.

Eupolis explains what a flatterer’s might say...

And when I catch sight of a man who is rich and thick, I at once get my hooks into him. If this moneybags happens to say anything, I praise him vehemently and express my amazement, pretending to find delight in his words.

Flattery is like soft soap…90 percent lye.

Only two groups of people fall for flattery—men and women.

A man’s body is remarkably sensitive. Pat him on the back and his head swells.

We do not hate flattery, any one of us – we all like it. —C. H. Spurgeon

Flattery corrupts both the receiver and the giver.—Edmund Burke

The Psalms speak of flattery...

There is nothing reliable in what they say. Their inward part is destruction itself. Their throat is an open grave; They flatter with their tongue. (Psalm 5:9)

They speak falsehood to one another; With flattering lips and with a double heart they speak. May the LORD cut off all flattering lips, The tongue that speaks great things (Psalm 12:2-3)

A man who flatters his neighbor Is spreading a net for his steps. (Proverbs 29:5)

Gossip is saying behind a person’s back what you would never say to his or her face; flattery is saying to a person’s face what you would never say behind his or her back. The Scriptures warn us repeatedly against flatterers, for they are destructive people who carry a legion of unwholesome motives.

Flattery - The aquatic creature called the blowfish has no particular value to the one who catches it—except that it may help to develop the angler's patience because it often seizes bait intended for better fish. The blowfish is unattractive; it has a large mouth and a wrinkled body that looks like worn-out leather. When you turn it over and tickle it, the flabby fish puffs up until it is swollen like a globe. People can be like that. A little flattery, a little tickling of their vanity and they swell up, giving the semblance of greatness. Pride inflates them, and they puff up like the blowfish. But there's nothing substantial about them; they are all air.

Francois Fenelon was the court preacher for King Louis XIV of France in the 17th century. One Sunday when the king and his attendants arrived at the chapel for the regular service, no one else was there but the preacher. King Louis demanded, “What does this mean?” Fenelon replied, “I had published that you would not come to church today, in order that your Majesty might see who serves God in truth and who flatters the king.”

C H Spurgeon wrote...

Praise is a thing we all love. I met with a man the other day who said he was impervious to flattery; I was walking with him at the time, and turning round rather sharply, I said, “At any rate, sir, you seem to have a high gift in flattering yourself, for you are really doing so, in saying you are impervious to flattery.” “You cannot flatter me,” he said. I replied, “I can, if I like to try; and perhaps may do so before the day is out.” I found I could not flatter him directly, so I began by saying what a fine child that was of his; and he drank it in as a precious draught; and when I praised this thing and that thing belonging to him, I could see that he was very easily flattered; not directly, but indirectly. We are all pervious to flattery; we like the soothing cordial, only it must not be labeled flattery; for we have a religious abhorrence of flattery if it be so called; call it by any other name, and we drink it in, even as the ox drinketh in water.

Men seldom flatter without a purpose; and they who listen to such music may expect to pay the piper. — Aesop

Rogers has this note on kolakeia...

Dio Chrysostom explains in a speech before the Emperor Trajan (of whom he says, “you delight in truth and frankness [aletheia kai parrhesia] rather than flattery and guile,” The Third Discourse, 2), “... even those flatterers who openly follow the business acknowledge that to play the flatterer is of all things most distasteful” (The Third Discourse, 16); then he adds, “Furthermore, flattery seems neither reputable nor honorable even when practiced to gain distinction, or from some other worthy motive. Nay, of all vices, I may say, flattery will be found to be the meanest” (aichisten) (The Third Discourse, 17). (Rogers, C L - originally by Fritz Rienecker: New Linguistic and Exegetical Key to the Greek New Testament. Zondervan. 1998)

McGee writes that...

Flattery disarms us—we really never know what to say. When people criticize me, I know what to say, but I never know what to say when someone flatters me. It disarms a person...

Paul never used flattery. There is a group of wealthy laymen across this country who are literally owned by the people who flatter them. If a Christian work or program doesn’t butter them up, they are not the least bit interested in helping that program financially. God pity the church or the work that must depend on men who require flattery and compliments before they will give their support to the work. I think this is one of the curses in the Christian church today. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson or Logos)

F B Meyer wrote about how tempting it is to flatter others...

When, by the grace of God, we have been delivered from grosser forms of sin, we are still liable to the subtle working of self, in our holiest and loveliest hours. It poisons our motive. It breathes decay on our fairest fruit-bearing. It whispers seductive flatteries into our pleased ears. It turns the spirit from its holy purpose, as the masses of iron on ocean steamers deflect the needle of the compass from the pole. So long as there is some thought of personal advantage, some idea of acquiring the praise and commendation of men, some aim at self-aggrandizement, it will be simply impossible to find out God's purpose concerning us. The door must be resolutely shut against all this, if we would hear the still small voice. All cross-lights must be excluded, if we would see the Urim and Thummim stone brighten with God's "Yes," or darken with His "No." Ask the Holy Spirit to give you the single eye, and to inspire in your heart one aim alone; that which animated our Lord, and enabled Him to cry, as He reviewed His life, "I have glorified Thee on the earth." Let this be the watchword of our lives, "Glory to God in the highest." Then our "whole body shall be full of light, having no part dark, as when the bright shining of a candle doth give light." (F. B. Meyer. Christian Living)

As you know (Paul repeatedly appeals to the reader's knowledge of the truthfulness of his words -- 1Thess 1:5; 2:1, 2:2, 2:5, 2:11; 4:2) -  Paul permits the facts to speak for themselves. He appeals to the Thessalonians' memory (note 1Thessalonians 1:5), which should allow them to testify regarding the fact that the missionaries' did not flatter nor exhibit a cloak of greed.

Know (1492) (oida) refers to having come to a perception or realization of something. Oida generally means to know intuitively or instinctively. To be acquainted with. To have information about. Know occurs repeatedly throughout Thessalonians, emphasizing that the Thessalonians had knowledge beyond a shadow of doubt

Pretext for greed - They did not use a "false front" nor "put on a mask to cover up greed". The KJV rendering of a cloak of covetousness paints a vivid picture. Paul declares that they did not preach among the Thessalonians in order to shake out whatever financial gain they could from them, trying all the while to hide this motive.  He emphasizes that he never misused his apostolic office in order to disguise, conceal or to hide avaricious designs. Instead, he reminded them that the missionaries had worked with their own hands while among the Thessalonians so as not to be a burden on any of them (see note 1Thess 2:9).

McGee writes that...

the cloak of covetousness (KJV) is a cloak of many colors. There are men who covet honor and fame and position. We need to search our hearts in order to uncover any covetousness there. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson or Logos)

The Pulpit Commentary writes that Paul is saying...

We did not use the gospel as a pretext to mask our real motive, which was covetousness, pretending to seek your spiritual good, whereas in reality we sought our own advantage. Paul could with perfect confidence appeal to his converts, and say,

I have coveted no one's silver or gold or clothes.  You yourselves know that these hands ministered to my own needs and to the men who were with me. In everything I showed you that by working hard in this manner you must help the weak and remember the words of the Lord Jesus, that He Himself said, 'It is more blessed to give than to receive. (Acts 20:33-34).

He was free from all sinister motives. (The Pulpit Commentary: New Testament; Old Testament; Ages Software  or Logos)

In Mark 12:40 (and Lu 20:47) below we see an example of the pretense made by the greedy Scribes who used their positions of authority to unjustly get money from widows.

Pretext (4392) (prophasis from prophaíno = to cause to shine before, to appear before, be apparent <> pró = before, + phaíno = to appear, to shine before. Vincent gives the origin as pro = before, in front of + phemi = to say, affirm) is that which is alleged as the cause, an allegation, plea. In other words it denotes something put forward for appearance to conceal what lies behind it.

In the NT it is used only in a bad sense and with the idea of an ulterior motive (Ulterior = going beyond what is openly said or shown and especially what is proper), of a falsely alleged motive or of an ostensible reason (or excuse). 

Our English word pretext is from the Latin, prætextum (prae = before + texere = weave) which described something woven in front of, with a view to concealment or deception or to screen or extend in front. Pretext is defined as a purpose or motive alleged or an appearance assumed in order to cloak the real intention or state of affairs. It suggests subterfuge and the offering of false reasons or motives in excuse or explanation (eg, using any pretext to get out of work).

It is an outward show or appearance, pretense or pretext to cover one's real intent or to hide the true state of things.  Here  it is an outward show or pretext used to conceal the reality "of greed".

Prophasis signifies the assuming of something so as to disguise one’s real motives.

Pretense refers to that which is professed rather than that which is one's heart intention or purpose.

Moulton and Milligan write that prophasis...

is the “ostensible reason” for which a thing is done, and generally points to a false reason as opposed to the true (Moulton, J. H., & Milligan, G. The Vocabulary of the Greek Testament. London: Hodder and Stoughton. 1930)

There are 7 uses of prophasis in the NT...

Matthew 23:14 Woe to you, scribes and Pharisees, hypocrites, because you devour widows' houses, even while for a pretense you make long prayers; therefore you shall receive greater condemnation.

Mark 12:40 who (referring to the Scribes) devour widows' houses, and for appearance's sake offer long prayers; these will receive greater condemnation. (Comment: The hypocritical scribes looked good outwardly but this was only a sham and a cover for their internal greed and selfishness)

Luke 20:47 who devour widows' houses, and for appearance's sake offer long prayers; these will receive greater condemnation. (Comment: Exactly how they would devour widows' houses is not clear but could refer to foreclosing on mortgages, to excessive charges for services rendered, to eating with widows as a forced hospitality, or to insisting that widows make generous contributions to the religious causes advocated by the scribes.)

John 15:22 "If I had not come and spoken to them, they would not have sin, but now they have no excuse for their sin.

Acts 27:30 And as the sailors were trying to escape from the ship, and had let down the ship's boat into the sea, on the pretense of intending to lay out anchors from the bow,

Philippians 1:18 (note) What then? Only that in every way, whether in pretense or in truth, Christ is proclaimed; and in this I rejoice, yes, and I will rejoice.

1 Thessalonians 2:5 For we never came with flattering speech, as you know, nor with a pretext for greed-- God is witness--

Bruce comments that...

Ostensibly the language might be above suspicion, but if its real purpose is the speaker's advantage, it is but a pretext for covetousness. That this was not so with Paul and his companions is known both to their converts and to God (God is witness) cf. v 10.  (Bruce, F F: 1 and 2 Thessalonians. Word Biblical Commentary. Dallas: Word, Incorporated. 1982 or Logos)

Hampton Keathley asks...

I wonder how many of the problems and heartaches going on in the church are rooted in this problem, the problem of people using ministry of all sorts to promote themselves and meet their own needs and wants whether financial or emotional. It is the problem of hidden agendas. (1Thessalonians 2:1-12 Compelling Example for Ministry)

Greed (4124) (pleonexia from pleíon = more + écho = have related to pleonektes = one who wants more, person covetous of something that others have) literally means to have more! It describes the insatiable and excessive desire to have more and thinks nothing of using another person or another’s property to gain its own ends. It is thus a strong desire to acquire more and more material possessions, especially that which is forbidden or to possess more things than other people have all irrespective of need. It is insatiable selfishness, greed, avarice, covetousness.

Thomas writes that

Pleonexia is self-seeking of all types, a quest for anything that brings self-satisfaction. It grows out of complete disinterest in the rights of others - an attitude foreign to Paul and his helpers.

Calvin rightly observes that...

Where greed and ambition hold sway, innumerable corruptions follow, and the whole man turns to vanity. These are the two sources from which stems the corruption of the whole of the ministry.

Adam Clarke warns...

Hear this, ye that preach the Gospel! Can ye call God to witness that in preaching it ye have no end in view by your ministry but his glory in the salvation of souls? Or do ye enter into the priesthood for a morsel of bread, or for what is ominously and impiously called a living, a benefice?...Is God witness that, in all these things, ye have no cloak of covetousness? . . . But woe to that man who enters into the labour for the sake of the hire! he knows not Christ; and how can he preach him?”

McGee sums up this section writing...

I love this passage. Paul could tell the Thessalonians, “When I came to you, I want you to know that I had no ulterior motives. I didn’t come for your offering. I didn’t come in order to shear your sheep. I came to give you the gospel and then to build you up in the faith. That was my motive.” With that kind of motive a person is really sailing on a marvelous sea. There may be storms, but the Lord will bring His servant through. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson or Logos)

Bruce writes that pleonexia which is translated as greed,

Covetousness or cupidity, the desire to get more as in 1Thes 4:6, is a vice especially reprobated in the NT. In Mark 7:22 it is included, along with fornication, adultery, murder and the like, among the evil things which come from within the human heart and convey real defilement (cf. 1 Cor 5:10, 11; 6:10). In Luke 12:15-21 the parable of the rich fool is told to encourage hearers to beware of all covetousness and to realize that a man's life does not consist in the abundance of his possessions.

In Colossians 3:5 (see note) and Ephesians 5:5 (see note) Greed is a form of idolatry. It is not merely the desire to possess more than one has, but to possess more than one ought to have, especially that which belongs to someone else. It is the sin of the man who has allowed full play to the desire to have what he should not have, who thinks his desires and appetites and lusts are the most important thing in the world, who sees others as things to be exploited, who has no god except himself and his desires (Barclay, William: New Testament Words:. Westminster John Know Press, 1964)  In 2 Cor 9:5; 12:17, 18 Paul rebuts the charge of pleonexia against himself and his colleagues in reference to the gathering of the Jerusalem relief fund, but there is nothing in the present context to suggest such a reference here. (Bruce, F F: 1 and 2 Thessalonians. Word Biblical Commentary. Dallas: Word, Incorporated. 1982 or Logos)

Barclay has an interesting historical comment writing that...

The first Christian book of order is called The Didache, The Teaching of the Twelve Apostles, and in it there are some illuminating instructions. “Let every apostles that cometh unto you be received as the Lord. And he shall stay one day and, if need be, the next also, but if he stay three days he is a false prophet. And when the apostle goeth forth, let him take nothing save bread, till he reach his lodging. But if he ask money, he is a false prophet.” “No prophet that ordereth a table in the Spirit shall eat of it, else he is a false prophet.” “If he that cometh is a passer-by, succour him as far as you can. But he shall not abide with you longer than two or three days unless there be necessity. But if he be minded to settle among you and be a craftsman, let him work and eat. But if he has no trade, according to your understanding, provide that he shall not live idle among you, being a Christian. But if he will not do this, he is a Christmonger: of such men beware.” (Didache, chapters 11 and 12). The date of The Didache is about A. D. 100. Even the Early Church knew the perennial problem of those who traded on charity. (Barclay, W: The Daily Study Bible Series. The Westminster Press or Logos)

GOD IS WITNESS: theos martus: (Romans 1:9; 9:1; Philippians 1:8; Galatians 1:20)

For God, Whom I serve in my spirit in the preaching of the gospel of His Son, is my witness as to how unceasingly I make mention of you (see note Romans 1:9)

For God is my witness, how I long for you all with the affection of Christ Jesus. (see note Philippians 1:8)

But I call God as witness to my soul, that to spare you I came no more to Corinth. (2Cor 1:23)

Witness (3144) (martus) refers to one Who has information or knowledge of something and hence can bring to light or confirm something. Three things are essential: (1) have seen with his own eyes what he attests; (2) be competent to relate it for others; (3) willing to testify truthfully. The present tense indicates believing is their lifestyle.

Men can judge the external conduct and can hear the flattering words but only God can know the real motive of their actions—He alone can discern a covetous heart seeing through the external cover or pretext.

Hiebert agrees writing that...

This appeal shows how important Paul regarded it to be cleared of this charge of gain-seeking in his ministry. Because the Thessalonians could judge only the character of the missionaries' conduct but could not know the true motives behind it, Paul takes his appeal directly to God who fully knows the hidden matters of the heart. Paul resorted to this appeal only in cases where human testimony was inadequate  (see notes Romans 1:9, Philippians 1:8) (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Vincent has an insightful note on God is witness writing that Paul is giving  a testimony to his conduct appealing to both human and divine witnesses...