Albert Barnes Commentary 1 Thessalonians 4:6

"that no man transgress, and wrong his brother in the matter: because the Lord is an avenger in all these things, as also we forewarned you and testified." — 1 Thessalonians 4:6 (ASV)

That no man go beyond. uperbainein. This word means to make to go over, as, for example, a wall or mountain; then, to overpass, that is, certain limits, to transgress; and then, to go too far, i.e., to go beyond right—hence to cheat or defraud. It is not used elsewhere in the New Testament. The idea of overreaching is what is implied in its use here.

And defraud. pleonektein. The marginal note is, oppress or overreach. This word properly means to have more than another; then to have an advantage; and then to take advantage of anyone, to circumvent, defraud, or cheat. It is rendered got an advantage (2 Corinthians 2:11); defraud (2 Corinthians 7:2; 1 Thessalonians 4:6); a gain (2 Corinthians 12:17–18). Compare for the use of the adjective (1 Corinthians 5:10–11; 1 Corinthians 6:10; Ephesians 5:5); and the noun (Mark 7:22; Luke 12:15; Romans 1:29; 2 Corinthians 9:5; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 2:5; 2 Peter 2:3, 14).

It is the word commonly used to denote covetousness. Taking advantage of is the idea it conveys here.

In any matter. The marginal note is, or "the." According to the reading in the margin, this would refer to the particular matter under discussion in 1 Thessalonians 4:3–5, namely, concupiscence. The meaning then would be that no one should be guilty of illicit intercourse with the wife of another—as Hammond, Whitby, Macknight, and Rosenmuller suppose that this is a prohibition of adultery, and there can be no doubt that it does include this.

But there is no reason why it should be confined to it. The Greek is so general that it may prohibit all kinds of fraud, overreaching, or covetousness, and may refer to any attempts to deprive another of his rights, whether it be the right he has in his property, or his rights as a husband, or his rights in any other respect. It is a general command not to defraud—not to act in a way that takes advantage of another, and in no way to deprive him of his rights.

Because that the Lord is the avenger of all such. Of all such as are guilty of fraud; that is, he will punish them. Compare Romans 12:19; Ephesians 6:9.

As we have also forewarned. Doubtless, when he was with them.