Forgiving and being forgiven is at the center of the Christian faith. Jesus, in the parable of the Unmerciful Servant, teaches Peter and us that our Heavenly Father forgives the impossible debt of sin we acquire because Jesus paid the debt of our sins. Now, we the forgiven are able to forgive others. Forgiving others is not easy. It required the death of His Son for God to forgive us. Even after God forgives us we often struggle to forgive each other. However, we can forgive others when, through faith, we begin to grasp and believe how completely God has forgiven all of us.
Ephesians 2:11-22 (Christ, through His forgiveness, glues us together as His people)
Matthew 18:21-35 (The Unmerciful Servant)