Jesus is adamant that we cannot receive God’s forgiveness unless we forgive others who have hurt us.
Far from being a simplistic admonition to “be nice,” this petition teaches us an invaluable truth about the spiritual life. If we are so caught up in our own grievances that we nurse grudges and refuse to forgive, our hearts are not open to God: we do not have the capacity to receive His mercy.
The Lord doesn’t want us to be turned in on ourselves, and our pain, but instead to give it to Him. We can do this by making an act of the will to forgive.
That doesn’t mean we can easily forget the offense, or trivialize it, or that we no longer feel the hurt. Rather, our decision to forgive is a step in our healing, which also serves to identify us with Christ.
If God Himself on the cross forgave those who were crucifying Him, how much more should we forgive our fellow frail human beings! By experiencing what it means to forgive an offense, we develop a greater appreciation for what God continually does for us. Despite our many failures to love Him, He is always eager to forgive us and begin anew.
For more, see Catechism paragraphs 2838-45.