A: 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 offend us.
A: Our sins are called debts, because we must satisfy God's justice for them either in this life or in the next.
A: 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.