To start, let's focus on God's response to human sinfulness, the hamartiological predicament of the moral rupture between ourselves and God.
If, following Maximus the Confessor, humanity fell from grace at the instant of creation then the first thing to note is that Christ is the "lamb slain from the foundation of the world" (Rev. 13.8). Other places in the New Testament also describe atonement as having been made before the beginning of time:
You know that you were ransomed from the futile ways inherited from your ancestors, not with perishable things like silver or gold, but with the precious blood of Christ, like that of a lamb without defect or blemish. He was destined before the foundation of the world, but was revealed at the end of the ages for your sake. (1 Peter 1.18-20)
Just as he chose us in Christ before the foundation of the world to be holy and blameless before him in love. He destined us for adoption as his children through Jesus Christ, according to the good pleasure of his will. (Eph 1.4-5)
Christ makes atonement before the foundation of the world. We were chosen in Christ before the foundation of the world.
So, while humanity falls in the first instant of creation, provision had already been made. The moral rupture was anticipated and had already been bridged. Though humanity fell into sin our guilt had already been removed.
This gift of grace, extended to us before the foundation of the world, becomes visible within history in the crucifixion of Jesus. To be clear, Calvary does not mark a change in God's affections toward us, wrath becoming favor. Such a notion violates core commitments in our doctrine of God. God is impassive toward human sin and does not require anything by way of sacrifice in order to forgive us. As we've seen, we've been forgiven before we even existed.
Calvary is, rather, a theophany. God's love is manifested within history at Golgotha. The atonement made before the foundation of the world is made visible at a particular time and place.
Given that humanity has been forgiven from the beginning of time, why would the theophany of Calvary be needed? Why make the atonement visible within history?
For four related reasons.
First, as described in the last post, when the creature stands before the holiness of God it will fall into despair. "Woe is me, I am lost!" Beyond fear, we also experience guilt and shame. The atonement becomes visible in human history to reassure the creature. When we despair over our sinfulness we can turn toward Calvary to behold our forgiveness. Praise and gratitude follow.
Second, humans are tempted to minimize the reality and severity of the moral rupture our sin has caused, the gravity of our situation. When the creature becomes indifferent or calloused toward sin it persists in drifting toward non-being, choosing nothingness over God. The visible display of the cost of sin at Calvary confronts the conscience and prompts contrition and repentance.
Third, love requires definition for proper emulation. To know love we must see it. 1 John 3.16: "This is how we know what love is: Jesus Christ laid down his life for us." There is also the imitation of self-offering love. Pay attention to second part of 1 John 3:16: "This is how we know what love is: Jesus Christ laid down his life for us. And we ought to lay down our lives for our brothers and sisters." The cross defines love and shows us how to love. The theophany of Calvary is a pedagogy of the heart.
Fourth, to draw us toward Himself God makes his love visible in history to evoke our ecstatic longings and desires. Our love seeks the object of its desire. The love made visible at Calvary evokes our love and draws us toward God. As Jesus says in John 12, "When I am lifted up from the earth I will draw all people to myself.”
For these reasons the atonement made from the foundation of the world becomes visible within history. Were atonement invisible we would grow anxious and uncertain and fall into doubt and despair. If the cost of sin was invisible we would become indifferent to our predicament and calloused toward sin. In a world of moral confusion and incoherence, we need a precise vision of love to emulate. And, finally, we need the love of God to become visible to evoke the longings and desires that draw us to God.