Last night with my Bible class we delve into the Sermon on the Mount as recorded in Matthew 5-7 and various parables of Christ found throughout the Gospels. One theme that is strikingly consistent is that of forgiveness. If we want our sins forgiven by the Father, we must forgive our brothers, in fact we vow this when we pray the Lord's Prayer. "Forgive us our debts/transgressions...as we forgive our debtors/trangessors". The parable about the unmerciful servant is the same way. St. Peter asks, "How many times shall I forgive my brother when he sins against me?" Christ answers "70 times 7." (Now, it should be asked, why didn't He just say "490 times"? Possibly He was making a connection to Dan. 9, where 70 sevens appears in context of God's forgiveness.)
The essence of Christianity, then, seems to be a concrete action: forgiveness. We can be forgiven by God for not understanding the hypostatic union, or for messing up predestination, or whatever. What cannot be forgiven, though, is our own unforgivingness. This is not because God is unable, far from it, but to not forgive is to consciously turn away from the likeness of God, who sends rain upon the just and the unjust. As C.S. Lewis puts it, "The gates of Hell are locked on the inside."
Forgive everyone for everything.