That no man go beyond and defraud his brother in any matter: because that the Lord is the avenger of all such, as we also have forewarned you and testified.
Go beyond - the bounds appointed by God, as to his brother. First, fornication is forbidden; then a holy use of its natural remedy affirmatively inculcated; lastly, the heinous sin, adultery, denounced (Ellicott).
Defraud - `overreach' (Alford).
In any matter, х en (G1722) too (G3588) pragmati (G4229)] - 'in the matter:' the matter now in question: the conjugal honour of his neighbour (1 Thessalonians 4:4); 1 Thessalonians 4:7 confirms this. "Brother" enhances the enormity. It is your Christian brother whom you wrong (cf. Proverbs 6:27-33).
The Lord - the coming Judge (2 Thessalonians 1:7-8).
Avenger of all such - `concerning all these things;' in all such wrongs against a neighbour's conjugal honour.
Testified, х diemarturametha (G1263)] - 'solemnly' (Ellicott); 'constantly testified' (Alford).