Matthew 18:15

ἔλεγξον αὐτὸν κ.τ.λ. ‘Rebuke him.’ See Leviticus 19:17, ‘Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke (ἐλέγξεις, LXX.) thy neighbour, and not suffer sin upon him’ (rather, ‘not bear sin on his account,’ ‘by bearing secret ill-will,’ Ephesians 4:26; or by ‘encouraging him to sin by withholding due rebuke.’ Speaker’s Commentary ad loc.).

ἐκέρδησας, ‘gained,’ i.e. won over to a better mind,—to Christ. Cp. 1 Corinthians 9:19-22, and 1 Peter 3:1. The aorist is of the action just past. If he shall have heard thee thou didst (at that moment) gain thy brother.