Albert Barnes on Matthew 18:21-22

Then came Peter, etc, The mention of the duty Mt 18:15 seeing a brother when he had offended us, implying that it was a duty to forgive him, led Peter to ask how often this was to be done.

Forgive him? To forgive is to treat as though the offence was not committed--to declare that we will not harbour malice, or treat unkindly, but that the matter shall be buried and forgotten.

Till seven times? The Jews taught that a man was to forgive another three times, but not the fourth. Peter more than doubled this, and asked whether forgiveness was to be exercised to so great an extent.

Until seventy times seven. The meaning is, that we are not to limit our forgiveness to any fixed number of times. See Gen 4:24. As often as a brother injures us, and asks forgiveness, we are to forgive him. It is his duty to ask forgiveness, Lk 17:4. If he does this, it is our duty to declare that we forgive him, and to treat him accordingly. If he does not ask us to forgive him, yet we are not at liberty to follow him with revenge and malice, but are still to treat him kindly, and to do him good, Lk 10:30-37.

(s) "forgive him" Mk 11:25, Lk 17:4, Col 3:13