John Calvin Commentary 1 Thessalonians 4:7

"For God called us not for uncleanness, but in sanctification." — 1 Thessalonians 4:7 (ASV)

For God hath not called us. This appears to be the same sentiment as the preceding one—that the will of God is our sanctification. There is, however, a little difference between them. For after having discussed correcting the vices of the flesh, he proves from the purpose of our calling that God desires this.

For he sets us apart to himself as his own possession. Again, he proves by contrast that God calls us to holiness, because he rescues us and calls us back from sexual immorality. From this he concludes that all who reject this doctrine reject not men, but God, the Author of this calling—a calling which completely collapses as soon as this principle of newness of life is overthrown.

Now, the reason he expresses himself so vehemently is that there are always licentious people who, while fearlessly despising God, ridicule all threats of his judgment and, at the same time, deride all commands for a holy and devout life. Such people must not be taught, but must be struck with severe rebukes as with a hammer blow.