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 James 1:18; James 3:14; John 8:32; 1. John 1:6; 1. John 3:18. 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 Luke 1:16, but intransitive often as Acts 9:35).