John Calvin Commentary 1 Thessalonians 1:2

"We give thanks to God always for you all, making mention [of you] in our prayers;" — 1 Thessalonians 1:2 (ASV)

We give thanks to God. He praises, as is his custom, their faith and other virtues, not so much, however, for the purpose of praising them, as to exhort them to perseverance.

For it is a significant encouragement to eager pursuit when we reflect that God has adorned us with remarkable gifts, that He may finish what He has begun, and that we have, under His guidance and direction, advanced in the right course in order to reach the goal. For as a vain confidence in those virtues, which mankind foolishly claims for themselves, puffs them up with pride and makes them careless and lazy for the future, so a recognition of the gifts of God humbles pious minds and stirs them up to earnest concern.

Therefore, instead of congratulations, he makes use of thanksgivings to remind them that everything in them that he declares to be worthy of praise is a gift from God. He also turns immediately to the future by mentioning his prayers. We thus see for what purpose he commends their previous life.