1 Thessalonians 4:1

In this chapter the apostle begins to remind them of their obligation of always striving to increase in virtue. Though he praises them through the whole epistle, he still thinks it necessary to warn them not to be surprised in uncleanness. He repeats what he had taught them before; first, that there is vengeance awaiting the workers of evil; and secondly, that the favour of God is the reward of those who deal with the brethren in simplicity, and preserve themselves from the defilements of the Gentiles. (St. Ambrose, Comment., on this place)