Brethren, if any of you do err from the truth - Either doctrinally and speculatively, by embracing error; or practically, by falling into sinful practices. Either of these may be called “erring from the truth,” because they are contrary to what the truth teaches and requires. What is here said does not appear to have any connection with what precedes, but the apostle seems to have supposed that such a case might occur; and, in the conclusion of the Epistle, he called their attention to the importance of endeavoring to save an erring brother, if such an instance should happen. The exhortation would be proper in addressing a letter to any church, or in publicly addressing any congregation.
And one convert him - This does not mean “convert him as a sinner, or regenerate him,” but turn him from the error of his way; bring him back from his wanderings; re-establish him in the truth, and in the practice of virtue and religion. So far as the word used here is concerned, ἐπιστρέψῃ epistrepsē he who had erred from the truth, and who was to be converted, may have been a true Christian before. The word means simply to turn, sc., from his way of error. See the notes at Luke 22:32.