Every action has an equal and opposite reaction. This is not a philosophical comment about human behaviour; it is one of the fundamental principles of the physical universe as described by Isaac Newton in 1687. It is known as Newton’s 3rd Law. The ground on which you are standing, or the chair on which you are sitting, is pushing you upwards with exactly the same force that your weight is pressing down. If it isn’t, then you are either sinking or taking off. It applies to all physical forces everywhere in the universe. They always work both ways.
Jesus was not a physicist. His genius was in the things of God and the things of human relationship. When unfolding his model for everyday prayer - the ‘Our Father’ or ‘Lord’s Prayer’ - Jesus drew attention to the fact that forgiveness, just like physical force, is fundamentally a two way process.
"Forgive us our trespasses, as we forgive those who trespass against us" does not set out a contractural arrangement of conditional forgiveness. Jesus was describing the very nature of forgiveness. To mimic Newton’s 3rd Law: every act of forgiving as an equal and opposite act of being forgiven. Forgiveness does not, and cannot, operate in just one direction.
This was clearly important to Jesus; he stressed the point immediately after the prayer. "For if you forgive others their trespasses, your heavenly Father will also forgive you; but if you do not forgive others, neither will your Father forgive your trespasses.” He isn’t telling us that God is picky about who he forgives. God’s not like that. Jesus is stating a universal principle about forgiveness. If you are not forgiving, you cannot be forgiven. And, to look at it from the other end, if you are truly forgiven, you cannot help forgiving.
Jesus observed this same principle when a prostitute interrupted a dinner party in order to cry over his feet. He said to the embarrassed guests, “Her sins, which were many, have been forgiven; hence she has shown great love. But the one to whom little is forgiven, loves little.”
Forgiveness cannot be hoarded up. It is explosive. If you choose to forgive someone this week, you will cause a chain reaction which cannot be stopped. Someone else, somewhere else, is going to end up being forgiven too. That’s how forgiveness works.