1 Thessalonians 4:3

For this is the will of God, even your sanctification
Which is another reason to enforce the above exhortation. "Sanctification" is internal or external. Internal sanctification is the work of the Spirit of God, and is a principle of spiritual life in the soul, a divine and spiritual light in the understanding, a flexion of the will to the will of God, and a settlement of the affections on divine things, and is an implantation of every grace in the heart. External sanctification arises from this, and lies in holiness of life and conversation; and is what is chiefly designed, as appears both by what goes before, and follows after: and this is "the will of God"; the will of his purpose and decree; for in the same decree that he wills the salvation of any by Jesus Christ, he also wills their sanctification in heart and life, and here and hereafter: and this is his approving will, or what is well pleasing in his sight, being agreeable to his nature, and divine perfections, particularly his holiness, in which he is glorious; and it is his will of command, and what he requires in his law, which is holy, just, and good, and perfectly agrees with the sound doctrine of the Gospel, and the revelation of his will in both.

That ye should abstain from fornication:
which is particularly mentioned, abstinence from it being a branch of external holiness; and because that this sin was common among the Gentiles, and not esteemed a sin by them; as also to observe to these Christians, that as simple fornication was not to be allowed of, much less other acts of uncleanness, as adultery, incest, sodomy, and the like, which were iniquities that greatly prevailed among the Heathens. The Syriac version renders it, "from all fornication"; on this subject the apostle enlarges in some following verses.