Since the Son was already loving and being hurt by our sin in the OT, he is the Lamb slain from the foundation of the world. So grace is not a payment made in a few hours on Good Friday to satisfy the wrath of God. This moves the atonement from a mere three hours on Good Friday to the very heart of God. The cross is therefore not a payment to satisfy the wrath of the Father but a visible expression in space and time of the eternal sacrificial love of God. That is not possible if he alone is the once and for all substitutionary sacrifice.
Genuine love cares about the freedom of the other.
And God's holiness is not to condemn but with a view to our eventual perfection. This is worked out in the Commentary on Romans. . We all agree that God is holy, but the holiness of love is not a necessity to condemn sinners to eternal damnation but the intention by all means and at great cost to perfect them for his heaven. One liberal view of the atonement is the moral influence theory which means that we are changed by the example of Christ's sacrifice. I prefer to say we are saved by grace alone and neither our forgiveness or transformation is in any way dependent on our goodness or moral efforts.