See the notes at Matthew 18:15, Matthew 18:21-22. "Trespass against thee." Sin against thee, or does anything that gives you an offence or does you an injury.
Rebuke - Reprove. Go and tell him his fault, and seek an explanation. Acquaint him with what has been the effect of his conduct, and the state of your feelings, that he may acknowledge his error and repent.

If thy brother trespass - See the notes on Matthew 18:21, Matthew 18:22.

(2) Take heed to yourselves: If thy brother trespass against thee, rebuke him; and if he repent, forgive him.
(2) Our reprehensions must be just and proceed from love and charity.

Take heed to yourselves..... Or to one another, that ye neither give, nor take offence. Take heed to your spirits, to your doctrines, walk, and conversation, that you give no offence to any, that you are not stumbled by what you shall see in, and meet with from others:
if thy brother trespass against thee; See Gill on Matthew 18:15.
rebuke him; privately, and proceed according to the rules there directed to; lay his sin before him; endeavour not only to convince him of the fact, but of the evil of it; how contrary to the will of God; how unbecoming the Gospel of Christ, and the profession he makes; how hurtful to himself, as well as injurious to his brother; and how such evils give the enemy occasion to reproach the saints, to speak evil of the ways of God, and blaspheme the name and doctrines of Christ, and harden sinners in their sins, as well as stumble weak Christians, and sadden the hearts of the righteous.
And if he repent; if he is made sensible of his evil, and is truly sorry for it, and ingenuously acknowledges it:
forgive him; the injury committed against a man's self; and pray to God for him, for an application of his pardoning grace and mercy to him; and comfort him with the hope of forgiveness with God, by the gracious promises and declarations of pardon made to such persons; drop all resentment and anger, and behave towards him with all sweetness of temper, and affability, and respect: and this is to be done immediately, as soon as a man repents: and so say the Jews (p);
"says R. Chanina bar Papa, whoever commits a thing, and repents of it, they forgive him directly; as it is said, Malachi 3:5 "and fear not me": lo, they that fear me, forgive immediately:''
such were reckoned good men, men fearing God.
(p) T. Bab. Chagiga, fol. 5. 1.

(See on Matthew 18:15-17; Matthew 18:21-22).

If thy brother sin, rebuke him. See notes on Matthew 18:15-22. Observe that kind rebuke, as well as forgiveness, is a duty.

Take heed to yourselves - That ye neither offend others, nor be offended by others. Matthew 18:15.