For if you forgive men their trespasses, your heavenly Father will also forgive you. But if do not forgive men their trespasses neither will your Father forgive your trespasses. Matthew 6:14-15
Then Peter came to Him and said, "Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?" Jesus said to him, "I do not say to you, up to seven times, but up to seventy times seven. Therefore the kingdom of heaven is like a certain king who wanted to settle accounts with his servants. And when he had begun to settle accounts, one was brought to him who owed him ten thousand talents. But as he was not able to pay, his master commanded that he be sold, with his wife and children and all that he had, and that payment be made. The servant therefore fell down before him, saying, "Master, have patience with me, and I will pay you all.’ Then the master of that servant was moved with compassion, released him, and forgave him the debt. "But that servant went out and found one of his fellow servants who owed him a hundred denarii; and he laid hands on him and took him by the throat, saying, "Pay me what you owe!’ So his fellow servant fell down at his feet and begged him, saying, "Have patience with me, and I will pay you all.’ And he would not, but went and threw him into prison till he should pay the debt. So when his fellow servants saw what had been done, they were very grieved, and came and told their master all that had been done. Then his master, after he had called him, said to him, "You wicked servant! I forgave you all that debt because you begged me. Should you not also have had compassion on your fellow servant, just as I had pity on you?’ And his master was angry, and delivered him to the torturers until he should pay all that was due to him."So My heavenly Father also will do to you if each of you, from his heart, does not forgive his brother his trespasses." Matthew 18:21-35
One of the most destructive forces in peoples lives today is unforgiveness toward another. It destroys relationships and destroys the bodies and minds of those who hold on to it. The root unforgiveness is pride and many would rather risk eternal separation from God than forgive those who have offended them. Eternal separation from God you would say? Why would a good God who is full of love and compassion send me to hell because I won’’t forgive my second grade teacher who blamed me for something I didn’t do or a family member who offended me one time? God does not want to send anyone to hell. Yes He is a loving God, but He is also righteous. When Jesus died on the cross for you, He took all of humanities sin upon Him, past, present, and future. Before you asked Him for forgiveness, the debt of sin you owed was more than you could pay. You were destined for hell. When you asked for forgiveness He forgave you that debt and gave you a new start.
If you do not forgive another like in the parable in Matthew 18 you are putting your self in serious danger. No amount of wrong that anyone has done to you could amount to what Christ forgave you. He showed you much mercy and because of that you are to show mercy to others.
But what if they do not forgive me? Pray for that person. Do all you can to reconcile. If you do your part you will be guiltless. Blessed are the merciful, for they shall obtain mercy. Matthew 5:7