Charles Ellicott Commentary Matthew 18:15

"And if thy brother sin against thee, go, show him his fault between thee and him alone: if he hear thee, thou hast gained thy brother." — Matthew 18:15 (ASV)

Moreover if thy brother shall trespass — Better, and if thy brother shall sin. A twofold train of thought is traceable in what follows. (1) The presence of “offenses” implies sin, and the question arises how each person is to deal with those sins that affect him personally. (2) The dispute in which the teaching recorded in this chapter had originated implied that the unity of the society, then represented by the Twelve, had for the time been broken. Sharp words may have been spoken among them, and the breach had to be healed.

Go and tell him his fault — The Greek is somewhat stronger: convict him of his fault, pressing it home on him in such a way as to reach his reason and his conscience . But this is to be done “between you and him alone.” Angry words spoken in the presence of others would fail to achieve that result. It is significant that the substance of the precept is taken from the passage that ends with, Thou shalt love thy neighbour as thyself (Leviticus 19:17–18).

You have gained your brother — The words in part derive their force from the subtle use of a word in one sense that people commonly associated with another. “Gain” of some kind, aimed at or wrongfully withheld, was commonly the origin of disputes and litigation. A person hoped to reap some profit by going to law. In the more excellent way our Lord points out, a person would, by sacrificing the lower gain, attain the higher, and win for God (see 1 Corinthians 9:19 and 1 Peter 3:1 for this aspect of the word) and for himself the brother with whom he had been at variance.