For. — The word further enforces the appeal to their memory: “Ye know what commandments... for this (you will recollect) is what God wants;” “a commandment given through the Lord Jesus,” being, of course, identical with “God’s will.”
Your sanctification. — In apposition to the word this. The mere conversion, justification, salvation of us are not the aim of God: He would have us holy. The general idea of sanctification passes however 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, e.g., 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. (See Hebrews 5:1-2.)