You have heard that it was said to the people long ago, 'Do not murder, and anyone who murders will be subject to judgment.' But I tell you that anyone who is angry with his brother will be subject to judgment. Again, anyone who says to his brother, 'Raca*,' is answerable to the Sanhedrin. But anyone who says, 'You fool!' will be in danger of the fire of hell. Therefore, if you are offering your gift at the altar and there remember that your brother has something against you, leave your gift there in front of the altar. First go and be reconciled to your brother; then come and offer your gift. Settle matters quickly with your adversary who is taking you to court. Do it while you are still with him on the way, or he may hand you over to the judge, and the judge may hand you over to the officer, and you may be thrown into prison. I tell you the truth, you will not get out until you have paid the last penny. [Matthew 5:21-26]
Also, he related the story of a man who had a great debt. The man he owed ordered that he and his wife and children be sold to repay the debt. The debtor begged and pleaded with the man to be merciful and mercy was granted. The debt was forgiven. A little later, this same forgiven man saw another man that owed him just a few dollars. He grabbed him by the neck and started to choke him, demanding that his money be paid right away. When man number three begged for forgiveness, it was denied. He was thrown in prison until he could repay the money. When the first man heard about this, he was angered and reinstated the original debt. (from Matthew 18)
The same is true with us. Jesus paid it all when he granted us salvation. We owe Him more than we could ever repay, and yet he asks for nothing. How dare we refuse to give forgiveness to others in the face of such mercy as we have been shown?
*That word, "Raca," according to Steven, means stupid or ignorant. Basically degrading someone as low as they can be degraded.