A.T. Robertson Commentary James 5:19

"My brethren, if any among you err from the truth, and one convert him;" — James 5:19 (ASV)

If any one among you do err (εαν τις εν υμιν πλανηθη). Third-class condition (supposed case) with εαν and the first aorist passive subjunctive of πλαναω, old verb, to go astray, to wander (Matthew 18:12), figuratively (Hebrews 5:2).

From the truth (απο της αληθειας). For truth see 1:18; 3:14; John 8:32; 1 John 1:6; 3:18f. It was easy then, and is now, to be led astray from Christ, who is the Truth.

And one convert him (κα επιστρεψη τις αυτον). Continuation of the third-class condition with the first aorist active subjunctive of επιστρεφω, old verb, to turn (transitive here as in Lu 1:16f. , but intransitive often as Ac 9:35).