For what thanksgiving. Not satisfied with a simple affirmation, he intimates how extraordinary is the greatness of his joy, by asking himself what thanks he can render to God; for by speaking thus he declares that he cannot find an expression of gratitude that can come up to the measure of his joy. He says that he rejoices before God, that is, truly and without any pretense.

For what thanks can we render to God again - That is, what expression of thanksgiving can we render to God that shall be an equivalent for the joy which your holy walk has furnished, or which will suitably express our gratitude for it.

What thanks can we render to God - The high satisfaction and uncommon joy which the apostle felt are strongly depicted in the language he uses. How near his heart did the success of his ministry lie! It was not enough for him that he preached so often, labored so hard, suffered so much; what were all these if souls were not converted? And what were all conversions, if those who embraced the Gospel did not walk steadily in the way to heaven, and persevere?

For what thanks can we render to God again or you,.... They had given thanks to God for them before, for their faith, love, and patience; see 1-Thessalonians 1:2 and now having received a further account of them, they looked upon themselves bound to give fresh thanks to God for them, for the increase of their faith and love, and for their perseverance, as they did; see 2-Thessalonians 1:3 since these are gifts of God's grace; but they knew not where to begin, what to render, and when to end: the phrase is much like that in Psalm 116:12 "what shall I render unto the Lord", &c.
for all the joy wherewith we joy for your sakes before our God: the joy they had was all joy, perfect and complete in its kind; see James 1:2 and it was not for themselves, on their own account, because of any worldly interest or advantage they had therein, but for the sake of these Thessalonians, whose spiritual welfare they rejoiced at: nor was their joy of a carnal nature, but spiritual; it was a joy in the Holy Ghost, and which was hearty and sincere: it was "before God our" God, their covenant God and Father; it was in his sight and presence, who sees and knows all things, all actions, and the principles of them. Though this last clause, "before our God", is in the Syriac version, placed at the beginning of the next verse, according to which, it refers, not to thanks or joy before God, but to prayer before him.

For what thanks--what sufficient thanks?
render . . . again--in return for His goodness (Psalm 116:12).
for you--"concerning you."
for all the joy--on account of all the joy. It was "comfort," 1-Thessalonians 3:7, now it is more, namely, joy.
for your sakes--on your account.
before our God--It is a joy which will bear God's searching eye: a joy as in the presence of God, not self-seeking, but disinterested, sincere, and spiritual (compare 1-Thessalonians 2:20; John 15:11).

What thanks can we render? How can we be thankful enough?
Night and day praying. An expression of his intense longing to see again a band of young converts so tenderly endeared to him.
And might perfect that which is lacking. Instruct them more fully in knowledge and duty. So recently heathen, they needed a wise teacher very much.
Now God himself. An appeal that he may be permitted to return to them. The prayer was granted in due time (Acts 20:1-2), though not for three or four years.
The Lord make you to increase. Whether he may come or not, his prayer is for their increase in love, the greatest of all graces.
To the end he may stablish your hearts. Love shed abroad in their hearts would have this effect, and fit them for every duty and trial. He that loveth supremely fulfill the whole law.