FOR WHAT THANKS CAN WE RENDER TO GOD FOR YOU IN RETURN:  tina gar eucharistian dunametha (1PPPI) to theo antapodounai (AAN) peri humon: (1Th 1:2,3; 2Samuel 7:18, 19, 20; Nehemiah 9:5; Psalms 71:14,15; 2Corinthians 2:14; 9:15)

What thanks can we render - The sense of this rhetorical question is that words simply cannot adequately express to God the thanks which filled Paul’s heart. His joy was overflowing every time he remembered these saints before his God.

Thanks (2169)(eucharista from eú = well + charízomai = to grant, give. It speaks of grateful language (to God, as an act of worship), thankfulness. Thanksgiving is only element of prayer that will continue forever. Expresses that which ought never to be absent from any of our devotions; namely, the grateful acknowledgment of past mercies, as distinguished from earnest seeking of future mercies.

Eucharista - 15x in 15v - Acts 24:3; 1 Cor 14:16; 2 Cor 4:15; 9:11f; Eph 5:4; Phil 4:6; Col 2:7; 4:2; 1 Thess 3:9; 1 Tim 2:1; 4:3f; Rev 4:9; 7:12. NAS = giving of thanks(3), gratefully*(1), gratitude(2), thankfulness(1), thanks(2), thanksgiving(4), thanksgivings(2).

Guzik writes that...

Some find it easy to rejoice in the material prosperity in the life of others, but Paul honestly rejoiced in the spiritual prosperity of others.

Hiebert explains that this is a rhetorical question...

expressive of deep emotion. Fully aware that his words are but a poor and inadequate formulation of the thanksgiving that is properly due, Paul yet attempts to give expression to the deep gratitude he feels. (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Render to God for you - Here for you means "concerning you" or "on your behalf". Note that Paul does not thank them but thanks God for them. While this should encourage them it provides no occasion for personal pride.

Render in return (467) (antapodidomi from antí = in turn + apodídomi = render <> from apo = from + didomi = give) means to give back in return for something received. The idea is to practice reciprocity with respect to an obligation.

It means to pay back something owed. Paul is saying  we as Christians owe to God joyful thanksgiving and praise for what He has done.

In a positive sense it means to repay, to recompense or to reward. In a negative sense it means to requite or  exact retribution. Requite implies a paying back according to one’s preference and often not equivalently.

Hiebert explains...

The effort to thank God "in return for" His gracious work translates the aorist active infinitive antapodounai (antapodidomi) "to repay, recompense, requite," and that as a definite act. The simple verb, meaning "to give," is compounded with two prepositions: anti, marking the idea of a return or exchange for something received, and apo, referring to something that is due "from" us as a debt; the thought is that of a full and complete return for the boon that has been received. No thanksgiving in their power is equivalent to the debt of gratitude they owe God because of the joy He has given them. This third outburst of thanksgiving for the readers (see notes 1Thessalonians 1:2; 2:13) is called forth by the news of their steadfastness under persecution. Their thanksgiving is Godward. That the Thessalonians stood steadfast is due not to the missionaries, or even to the converts, hut to God Himself who has upheld them under the storm of persecution. Paul viewed all spiritual blessings :is coming ultimately from God. What by human standards would have been regarded as a triumph for the missionaries, Paul humbly acknowledges to be the work of God. Let Christian workers beware of taking credit for results only God can produce. (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Antapodidomi is used 53 times in the Septuagint (LXX) (Gen. 44:4; 50:15; Lev. 18:25; Deut. 32:6, 35, 41, 43; Jdg 1:7; 16:28; 1 Sam. 24:17; 25:21; 2 Sam. 3:39; 19:36; 22:21; 1 Ki. 2:44; 2 Ki. 9:26; 2 Chr. 32:25; Job 21:19, 31; Ps. 7:4; 18:20, 24; 31:23; 35:12; 38:20; 41:10; 103:10; 116:12; 119:17; 137:8; 138:8; 142:7; Prov. 19:17; 25:22; Isa. 35:4; 59:18; 63:7; 66:4, 6; Jer. 16:18; 18:20; 50:29; 51:6, 24, 56; Hos. 4:9; 12:2, 14; 14:2; Joel 2:25; 3:4, 7; Ob 1:15; Zech. 9:12). Here are some representative uses...

Genesis 50:15 When Joseph's brothers saw that their father was dead, they said, "What if Joseph should bear a grudge against us and pay us back in full for all the wrong which we did to him!"

Deuteronomy 32:35 'Vengeance is Mine, and retribution (Lxx = antapodidomi = "I will repay"), In due time their foot will slip; For the day of their calamity is near, And the impending things are hastening upon them.'

2Samuel 22:21 "The LORD has rewarded (Lxx = antapodidomi) me according to my righteousness; According to the cleanness of my hands He has recompensed (Lxx = antapodidomi) me.

Psalm 31:23 O love the LORD, all you His godly ones! The LORD preserves the faithful, And fully recompenses the proud doer.

Psalm 103:10 He has not dealt with us according to our sins, Nor rewarded us according to our iniquities.

Psalm 116:12 What shall I render to the LORD For all His benefits toward me?

Proverbs 25:22 For you will heap burning coals on his head, And the LORD will reward you.

Joel 2:25 "Then I will make up to (Lxx = antapodidomi) you for the years That the swarming locust has eaten, The creeping locust, the stripping locust, and the gnawing locust, My great army which I sent among you.

Obadiah 1:15 "For the day of the LORD draws near on all the nations. As you have done, it will be done to you. Your dealings will return (Lxx = antapodidomi - literally the Greek reads "your reward will be recompensed") on your own head.

Antapodidomi is used 7 times...

Luke 14:14 and you will be blessed, since they do not have the means to repay you; for you will be repaid at the resurrection of the righteous.

Romans 11:35 (note) Or who has first given to Him that it might be paid back to him again?

Romans 12:19 (note) Never take your own revenge, beloved, but leave room for the wrath of God, for it is written, "Vengeance is Mine, I will repay," says the Lord.

1Thessalonians 3:9 (note) For what thanks can we render to God for you in return for all the joy with which we rejoice before our God on your account,

2 Thessalonians 1:6 For after all it is only just for God to repay with affliction those who afflict you

Hebrews 10:30 (note) For we know Him who said, "Vengeance is Mine, I will repay." And again, "The Lord will judge His people."

FOR ALL THE JOY WITH WHICH WE REJOICE BEFORE OUR GOD ON YOUR ACCOUNT:  epi pase te chara e chairomen (1PPAI) di' humas emprosthen tou theou hemon: (1Thes 3:7,8; 2:19) (Deuteronomy 12:2,18; 16:11; 2Samuel 6:21; Psalms 68:3; 96:12,13; 98:8,9)

For all the joy with which we rejoice - God's joy sometimes comes to us because of other Christians and their faithfulness to Christ. Literally Paul says

Upon all the joy wherewith we are rejoicing

Joy (5479) (chara) is a feeling of inner gladness, delight or rejoicing. Secular dictionaries define joy as the emotion evoked by well-being, success, or good fortune or the emotion evoked by the prospect of possessing what one desires. The world's definition of joy is therefore virtually synonymous with the definition of happiness, for both of these "emotions" are dependent on what "happens".

Rejoice (5463)(chairo) means to be "cheer" full or full of cheer, to be delighted, to be calmly happy or to be well-off in one's emotional status. Chairo was frequently used in ancient Greece as  a greeting or address in the imperative (command) implying a wish for well-being (something like "welcome", "good morning").

Chairo - 74x in 68v - Matt 2:10; 5:12; 18:13; 26:49; 27:29; 28:9; Mark 14:11; 15:18; Luke 1:14, 28; 6:23; 10:20; 13:17; 15:5, 32; 19:6, 37; 22:5; 23:8; John 3:29; 4:36; 8:56; 11:15; 14:28; 16:20, 22; 19:3; 20:20; Acts 5:41; 8:39; 11:23; 13:48; 15:23, 31; 23:26; Rom 12:12, 15; 16:19; 1 Cor 7:30; 13:6; 16:17; 2 Cor 2:3; 6:10; 7:7, 9, 13, 16; 13:9, 11; Phil 1:18; 2:17f, 28; 3:1; 4:4, 10; Col 1:24; 2:5; 1 Thess 3:9; 5:16; Jas 1:1; 1 Pet 4:13; 2 John 1:4, 10f; 3 John 1:3; Rev 11:10; 19:7. NAS = am glad(1), glad(7), gladly(1), greeted*(1), greeting(2), greetings(4), hail(4), joyfully(1), make(1), rejoice(33), rejoiced(8), rejoices(2), rejoicing(10).

In this passage, the present tense indicates that their joy was a continuing, supernatural, Spirit given fruit (Gal 5:22-note), not a passing emotional reaction as one sees in those who are merely "happy" where one is happy when things are going well but not happy when things are not going well.

Hiebert writes that

The use of both the noun and verb underlines the reality of their joy, while "all" brings out the superlative quality of the joy. Because of the nature and scope of their joy, they owe God a greater debt of thanks than they can express. Theirs is a continuing joy, as the present tense of the verb indicates. It counterbalances "all our distress and persecution" (1Th 3:7). (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Before (1715)(emprosthen from en = in, + prósthen = in front of, before from prós = toward, + syllabic suffix –then denoting direction, a place) means before, in front of., in presence of, sight of, used of place or position only.

Emprosthen - 48x in 45v - Matt 5:16, 24; 6:1f; 7:6; 10:32f; 11:10, 26; 17:2; 18:14; 23:13; 25:32; 26:70; 27:11, 29; Mark 2:12; 9:2; Luke 5:19; 7:27; 10:21; 12:8; 14:2; 19:4, 27f; 21:36; John 1:15, 30; 3:28; 10:4; 12:37; Acts 10:4; 18:17; 2 Cor 5:10; Gal 2:14; Phil 3:13; 1 Thess 1:3; 2:19; 3:9, 13; 1 John 3:19; Rev 4:6; 19:10; 22:8. NAS = ahead(5), before(28), front(4), higher rank(2), presence(4), sight(3).

On your account - Paul again stresses that the truth about the steadfastness of the Thessalonian converts is the catalyst that brings about their continual rejoicing with all joy. Note the radical change from all our distress and affliction (1Th 3:7-note)  to all the joy on account of the steadfastness of the Thessalonians. Paul is modeling for us the discipline of giving thanks in all things (distress and affliction, cp note 1Th 5:18-note). How does this supernatural reaction relate to 1Th 1:6-note? Clearly we see the key to rejoicing in tribulations and giving thanks in all things is being controlled by the Holy Spirit, continually learning to lean on His strength and not our own understanding (cp Proverbs 3:5, 6) Joy is a fruit of a Spirit filled life and here was "fertilized" by the good news brought by Timothy.

Hiebert writes that...

That they are expressing it before "our God," the God with whom they stand in intimate personal relationship, indicates its purity. It is a joy that is given free rein in the presence of God without embarrassment, for it is uncontaminated by personal selfishness or worldly motives. We have here another instance of Paul's practice of lifting everything that came into his life, whether sad or glad, into the presence of God. Thus he lived in the sense of God's presence with him. (Ibid)

The psalmist writes...

But let the righteous be glad; let them exult before God; Yes, let them rejoice with gladness. (Ps 68:3)

Spurgeon comments The presence of God on the throne of grace is an overflowing source of delight to the godly; and let them not fail to drink of the streams which are meant to make them glad.  The courtiers of the happy God should wear the garments of gladness, for in his presence is fulness of joy. That presence, which is the dread and death of the wicked, is the desire and delight of the saints. Let them dance with all their might, as David did, for very joy. No bounds should be set to joy in the Lord. "Again, I say, rejoice," says the apostle, as if he would have us add joy to joy without measure or pause. When God is seen to shine propitious from above the mercy seat in the person of our Immanuel, our hearts must needs leap within us with exultation, if we are indeed among those made righteous in his righteousness, and sanctified by his Spirit. Move on, O army of the living God, with shouts of abounding triumph, for Jesus leads the van.