Charles Ellicott Commentary 1 Thessalonians 4:3

"For this is the will of God, [even] your sanctification, that ye abstain from fornication;" — 1 Thessalonians 4:3 (ASV)

For.—The word further enforces the appeal to their memory: You know what commandments were given... for this (you will recall) is what God wants; “a commandment given through the Lord Jesus” is, of course, identical with “God’s will.”

Your sanctification.—This is in apposition to the word this. Our mere conversion, justification, and salvation are not God’s aim: He wants us to be holy. The general idea of sanctification, however, passes here, as the following clauses show, into the more limited sense of purification.

Fornication.—The word is often used in late Greek for any kind of impurity, as, for example, in 1 Corinthians 5:1, of incest; but here it must be understood in its strict sense. To the Gentile mind, while the wickedness of adultery or incest was fully recognised, it was a novelty to be told that fornication was a “deadly sin;” hence the strange connection in which it stands in the Synodal letter to the Gentile churches (Acts 15:20; Acts 15:29; Acts 21:25). This consideration also makes it easier to understand how St. Paul can praise these Gentile Thessalonians so heartily, although they need earnest correction on this vital point. It is a true instance of the sacerdotal metriopathy (or, compassionate consideration) towards the ignorant and deceived .