I'm not going to offer a full defense of this claim in this post. Frankly, I think it's so obviously true that it is only denied because the commandment is so difficult to keep. But I did want to look at one argument against it, an argument I think has a curious feature. (The argument was offered in the middle of this very long exchange on the place of anger in the Christian life.)
The counter-argument can be expressed this way: God Himself doesn't forgive wrongdoers who don't seek forgiveness, and God doesn't hold us to a higher standard than that to which He holds Himself. Therefore, we don't need to forgive wrongdoers who don't seek forgiveness.
What I find interesting here is the idea that forgiving people regardless of whether they seek forgiveness -- call it "unconditional forgiveness" -- is regarded as a "higher standard" than forgiving people only if they seek forgiveness. To me, a "higher standard" is a better standard, one closer to the ideal. So -- looking at the first sentence in this post -- I would certainly agree that unconditional forgiveness is a higher standard than conditional forgiveness.
What I don't agree with is that there is a higher standard than God's own standard. I mean, how could there be? Nothing can be done more perfectly than what God does. God is the ideal, He defines the standard (and not just by declaration, but by His very existence).
So how can unconditional forgiveness be a higher standard than conditional forgiveness if the latter is God's own standard?
The short answer is that we are not God, and His ways are not our ways, so His standards are not our standards. What makes unconditional forgiveness higher, instead of simply stricter or more demanding, is that God commands it.
The medium answer is that, though unconditional forgiveness is a higher standard expressed positively, in terms of what we must do, it is a lower standard expressed negatively, in terms of what we must not do. "Forgive all who wrong you" is equivalent to "Do not judge who is to be forgiven and who is not." What is better from the human perspective is worse from the divine perspective, and since we are not God we must not claim the right to judge that belongs to God alone.
The long answer begins with the observation that we are called to be perfect as our Father is perfect, which means we are called to the Divine standard, so unconditional forgiveness and Divine justice are somehow compatible.
I think it's suggestive that, in the Sermon on the Mount, the words
I say to you, love your enemies, and pray for those who persecute you, that you may be children of your heavenly Father, for He makes His sun rise on the bad and the good, and causes rain to fall on the just and the unjust.
come so soon before
Forgive us our debts, as we forgive our debtors.
The ongoing mercy and forgiveness God offers (and therefore we are to offer) to the just and the unjust is, perhaps, the projection of eternal Divine justice onto time. We must, then, forgive without concern for who "deserves" forgiveness, because not only are we not fit to make that judgment, now is not the time for that judgment to be made.
And that is the end of the beginning of the long answer.