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.

Err from the truth - Stray away from the Gospel of Christ; and one convert him - reclaim him from his error, and bring him back to the fold of Christ.

(12) Brethren, if any of you do err from the truth, and one (k) convert him;
(12) The taking away of an objection: all rebukes are not condemned, seeing that on the contrary there is nothing more acceptable to God than to call into the holy way, a brother that was wandering out of the way.
(k) Has called him back from his way.

Brethren, if any of you do err from the truth,.... Either from Christ, who is the truth, by departing from him, forsaking his ways, worship, and ordinances; or from the Scriptures of truth, not speaking according to them, and embracing notions that are contrary unto them; or from the Gospel, the word of truth, from the doctrine of faith, and from uprightness of life and conversation, after having made a profession of Christianity; for this is to be understood of one who has embraced the Christian religion, become a member of a church, and has walked in the path of truth and holiness, but now fallen into error, either in principle, or in practice, or both:
and one convert him; or turn him from his error, to truth again; for this designs not first conversion, or the turning of a sinner from darkness to light, from the power of Satan to God, and from the evil of a man's heart and ways and from a dependence on his own righteousness, to the Lord Jesus Christ, to look to him for righteousness, life, and salvation, which is wholly and entirely God's work, and not man's; but conversion after backslidings; for a restoration from a fallen condition is sometimes so called, Psalm 51:1 and which one brother may be an instrument of to another, by showing him, and setting before him, the evil of his errors, whether in principle or practice; and by instructing him in the doctrines of the Gospel, and in the duties of religion; and by reproving him in meekness, and according to the rules of Christ; which means are sometimes blessed for the gaining of such; and which may be called conversion: and also, this is sometimes done by praying for him; and which seems chiefly to be intended here; for from praying for the healing of the diseases of the body, the apostle proceeds to encourage the saints to pray for one another, for the healing of the diseases of the mind; and suggests, that if prayer avails to the one, it may to the other; and which is the most desirable, and the greatest blessing, as follows.

It is no mark of a wise or holy man, to boast of being free from error, or to refuse to acknowledge an error. And there is some doctrinal mistake at the bottom of every practical mistake. There is no one habitually bad, but upon some bad principle. This is conversion; to turn a sinner from the error of his ways, not merely from one party to another, or from one notion and way of thinking to another. There is no way effectually and finally to hide sin, but forsaking it. Many sins are hindered in the party converted; many also may be so in others whom he may influence. The salvation of one soul is of infinitely greater importance than preserving the lives of multitudes, or promoting the welfare of a whole people. Let us in our several stations keep these things in mind, sparing no pains in God's service, and the event will prove that our labour is not in vain in the Lord. For six thousand years He has been multiplying pardons, and yet his free grace is not tired nor grown weary. Certainly Divine mercy is an ocean that is ever full and ever flowing. May the Lord give us a part in this abundant mercy, through the blood of Christ, and the sanctification of the Spirit.

The blessing of reclaiming an erring sinner by the mutual consent and intercessory prayer just recommended.
do err--more literally, "be led astray."
the truth--the Gospel doctrine and precepts.
one--literally, "any"; as "any" before. Everyone ought to seek the salvation of everyone [BENGEL].

As if he had said, I have now warned you of those sins to which you are most liable; and, in all these respects, watch not only over yourselves, but every one over his brother also. Labour, in particular, to recover those that are fallen. If any one err from the truth - Practically, by sin.