Matthew 6:14-15

For if you forgive others their trespasses (their reckless and willful sins), your heavenly Father will also forgive you. But if you do not forgive others (nurturing your hurt and anger with the result that it interferes with your relationship with God) then your Father will not forgive your trespasses.

The devil uses offense for the sinner or the saint to derail a connection to God. If we become blinded by offense and our feelings, we lock our focus on our minds, wills, and emotions, leaving God out of our direction. It becomes a spiral of self, and love gets choked out when we focus on self. Forgiveness is vital for us to remain in close contact with God. God is unable to reach the heart which is blocked by unforgiveness.