John Calvin Commentary 1 Thessalonians 4:8

"Therefore he that rejecteth, rejecteth not man, but God, who giveth his Holy Spirit unto you." — 1 Thessalonians 4:8 (ASV)

Who has also given. So that he may more effectively turn the Thessalonians away from such contempt and obstinacy, he reminds them that they have been endowed with the Spirit of God for these reasons:

1. that they may distinguish what proceeds from God;
2. that they may make a fitting distinction between holiness and impurity;
3. and that, with heavenly authority, they may pronounce judgment against every kind of unchastity—a judgment that will fall upon their own heads unless they keep themselves separate from its defilement.

Therefore, however much wicked men may ridicule all instructions concerning a holy life and the fear of God, those who are endowed with the Spirit of God have a very different testimony sealed upon their hearts. We must therefore be careful, lest we extinguish or obliterate it.

This might also refer to Paul and the other teachers, as if Paul had said that they condemn unchastity not from human perception, but pronounce by God's authority what His Spirit has suggested to them. I am inclined, however, to include both. Some manuscripts have the second person—you, which restricts the gift of the Spirit to the Thessalonians.