For if you forgive men their trespasses who trespass against you, your Heavenly Father will also forgive you. But if you forgive not men their trespasses, neither will your Heavenly Father forgive you your trespasses. (Matthew 6:14-15)
God knows the thoughts and intents of our heart and mind. He knows all of our works. There is nothing known save he knows it. We as well as this creation are created by him through Christ. We agreed before coming here to be subject to the laws of this creation. Though sinless, Christ suffered the sins and errors of all mankind and by this gained the power and the right to judge all mankind.
God is of a forgiving nature. He doesn't want to punish. But he is the personification of Justice as well as Mercy. Therefore if we do not forgive, he cannot forgive us. If we cannot be forgiven, we remain unclean and unable to enter the kingdom of Heaven. If we cannot enter the kingdom of Heaven, we must at death dwell with the damned spirits in hell. If we do forgive, God can then forgive us and satisfy the demands of justice via Christ's suffering on our behalf.
It doesn't matter whether the person who trespassed against us does or does not repent. We are to forgive.
