Albert Barnes 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. The word brother here evidently means a fellow believer. Christians are called fellow believers because they belong to the same redeemed family—having God as their common Father—and because they are united in the same feelings, purposes, and destiny.

Trespass against thee. That is, to injure you in any way, by words or conduct. The original word means to sin against you. This may be done by injuring the character, person, or property.

Go and tell him his fault. This was required under the Law (Leviticus 19:17). In the original, it is, “go and reprove him.” Seek an explanation of his conduct; and if he has done wrong, administer a friendly and brotherly reproof. This is required to be done alone:

1. That he may have an opportunity to explain it. In nine cases out of ten, when someone supposes he has been injured, a little friendly conversation would set the matter right and prevent difficulty.
2. That he may have an opportunity to acknowledge his offence or make reparation if he has done wrong. Many would be glad of such an opportunity, and it is our duty to furnish it by calling on them.
3. That we may admonish them for their error if they have harmed the cause of religion. This should not be publicized widely. It can do no good; it causes injury. It is what the enemies of religion wish. Christ is often wounded in the house of His friends, and religion, as well as an injured brother, often suffers when such faults are spread before the world.

Thou hast gained thy brother. To gain sometimes means to preserve or to save (1 Corinthians 9:19). Here it means you have preserved him or restored him to be a consistent Christian. Perhaps it may also include the idea that you have reconciled him—you have gained him as a Christian brother.