Matthew 6:12 
and forgive us our debts, 
    as we also have forgiven our debtors. 
 
Matthew 18:35 
“So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart.” 
We’ve established in a previous post that our relationship with God is tied to our relationship with others. This verse takes the idea even further. It ties our own forgiveness to the standard we extend to others. Why would God do this? What makes unforgiveness, an issue between two people, into a sin against God?
Unforgiveness is a sin against God because our relationship with God is not separate from our relationship with other people. When God declared it not good that man be alone, He was not simply referring to marriage. We discussed previously how the Lord’s Prayer is a group prayer (“our” being the operative pronoun). The human experience is by design one of community. We are cumulatively, not individually, the body of Christ.
More than that, however, unforgiveness is idolatry. It is an attempt to disbar God and make ourselves the ultimate judge. Judgment is God’s, He shall punish wrongs, not us. When we choose not to forgive, we are declaring that He is not good enough at His job.
When we place our trust in God, we must do so for all His roles, not just as savior. Forgive others, for He has forgiven us.