For what thanks can we render to God again for you, for all the joy wherewith we joy for your sakes before our God;
What - what sufficient thanks?
Render to God again - in return for His goodness (Psalms 116:12).
For, [ peri (G4012 ): concerning] you, for (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, as in the presence of God, which will bear His searching eye; not self-seeking, but disinterested, sincere, and spiritual (cf. 1 Thessalonians 2:20; John 15:11).