Adam Clarke on 1 Thessalonians 4:3

This is the will of God, even your sanctification - God has called you to holiness; he requires that you should be holy; for without holiness none can see the Lord. This is the general calling, but in it many particulars are included. Some of these he proceeds to mention; and it is very likely that these had been points on which he gave them particular instructions while among them.

That ye should abstain from fornication - The word πορνεια, as we have seen in other places, includes all sorts of uncleanness; and it was probably this consideration that induced several MSS., some versions and fathers, to add here πασης, all. Directions of this kind were peculiarly necessary among the Greeks, and indeed heathens in general, who were strongly addicted to such vices.