The Bible teaches that although man has sinned tremendously against God, He is willing to forgive if only man will have the humility to draw near and admit his sin. If this promise is available for man, then God expects man to offer the same forgiveness to his fellowmen.
The Lord Jesus Christ preached the parable concerning a servant who refuses to forgive the money owed to him by a fellow servant, even though he already received forgiveness from their master. Jesus teaches that if a man seeks forgiveness, he should also be ready to forgive.
Unforgiveness is not a good quality especially for a Christian, someone who has already received payment and forgiveness for his sins. Another problem with unforgiveness is that it grows and yields more evil fruits such as bitterness and hatred.