In this petition, we ask God to forgive us our sins, as we forgive those who sin against us. Our sins are called debita, i.e. "debts," because we ought to satisfy the justice of God, either in this world or in the next.
If we do not forgive our neighbors, we have no right to hope that God will forgive us, inasmuch as we condemn ourselves when we ask God to forgive us as we forgive our neighbors.
309. What do we ask in the Fifth Petition; And forgive us our trespasses, as we forgive them that trespass against us?
In the Fifth Petition: And forgive us our trespasses as we forgive them that trespass against us, we ask God to pardon us our sins as we pardon those who sin against us.
310. Why are our sins called debita or "debts"?
Our sins are called debita or "debts," because we must satisfy God's justice for them, either in this life or in the next.
311. Can those who do not forgive their neighbor hope that God will pardon them?
Those who do not forgive their neighbor have no reason to hope that God will pardon them; especially since they condemn themselves when they ask God to forgive them as they forgive their neighbor.