Directly after Jesus told Peter to forgive a brother four-hundred and ninety times, He told a parable to help Peter understand what He meant:
For this reason the kingdom of heaven may be compared to a certain king who wished to settle accounts with his slaves. And when he had begun to settle them, there was brought to him one who owed him ten thousand talents [This would have been equivalent to over 5,000 years of wages for the average laborer in Jesus’ day.] But since he did not have the means to repay, his lord commanded him to be sold, along with his wife and children and all that he had, and repayment to be made. The slave therefore falling down, prostrated himself before him, saying, “Have patience with me, and I will repay you everything.” And the lord of that slave felt compassion and released him and forgave him the debt. But that slave went out and found one of his fellow slaves who owed him a hundred denarii [equivalent to one hundred day’s wages]; and he seized him and began to choke him, saying, “Pay back what you owe.” So his fellow slave fell down and began to entreat him, saying, “Have patience with me and I will repay you.” He was unwilling however, but went and threw him in prison until he should pay back what was owed. So when his fellow slaves saw what had happened, they were deeply grieved and came and reported to their lord all that had happened. Then summoning him, his lord said to him, “You wicked slave, I forgave you all that debt because you entreated me. Should you not also have had mercy on your fellow slave, even as I had mercy on you?” And his lord, moved with anger, handed him over to the torturers until he should repay all that was owed him. So shall My heavenly Father also do to you, if each of you does not forgive his brother from your heart (Matt. 18:23-35).
Notice that the first slave was forgiven because he asked his master for it. Then notice that the second slave also humbly asked the first slave for forgiveness. The first slave did not grant the second slave what he had been granted, and that is what so angered his master. This being so, would Peter have thought that Jesus was expecting him to forgive an unrepentant brother who never asked for forgiveness, something not illustrated at all by Jesus’ parable? That seems unlikely, and even more so since Jesus had just told him to treat an unrepentant brother, after he had been properly confronted, like a Gentile and tax collector.
It seems even more unlikely that Peter would have thought that he was expected to forgive an unrepentant brother in light of the punishment that Jesus promised us if we don’t forgive our brothers from our hearts. Jesus promised to reinstate all of our formerly-forgiven debt and hand us over to the torturers until we repay what we can never repay. Would that be a just punishment for a Christian who does not forgive a brother, a brother whom God does not forgive either? If a brother sins against me, he sins against God, and God does not forgive him unless he repents. Can God justly punish me for not forgiving someone whom He does not forgive?