His justice must be satisfied. God's wrath over the personal and legal aspects of man's sin must be appeased. Christ, by His active and passive obedience had to live the life God's holiness required and die the death this justice exacted. Mercy and wrath kissed at the cross. If I were to punch you in the nose, then ask someone sitting next to you to forgive me, that wouldn't do.
Forgiveness is a transaction that always involves the two parties involved in the offense.
In this way, all is satisfied that should be. In contrast, the liberal view amounts to little more than a toleration or a condoning of sin. To live forgiveness is to give wholehearted acceptance to others. Forgiveness is acceptance with no exception. To separate Christ's prayer on the cross from His crucifixion as Augsburger seems to do in this place is a tragic mistake.