"Why must we forgive all offenses? Because we have been forgiven all offenses by the One whom we offend in all offenses. ("Whatever you do to one of the least of theses, my family, you do to me.")
And because that One tells us that we cannot be forgiven unless we forgive.
Why? Not because God withholds it. He doesn't. But we cannot receive it, even though He gives it, when the hands of our souls are closed.
When we don't forgive others, we make them our masters. When we chew on others' faults, we make them the masters of our misery.
If we forgive only the forgivable and not the unforgivable, if we proportion our love to dessert, then we subject love to justice. And that is idolatry, for God is love. Justice is only love's backup. When love is gone, justice is needed to protect us from each other."
- Peter Kreeft, Before I Go. Lanham: Sheep & Ward, 2007.