Anglican pastor John Stott:
The concept of substitution may be said, then, to lie at the heart of both sin and salvation. For the essence of sin is man substituting himself for God, while the essence of salvation is God substituting himself for man. Man asserts himself against God and puts himself where only God deserves to be; God sacrifices himself for man and puts himself where only man deserves to be. Man claims prerogatives that belong to God alone; God accepts penalties that belong to man alone.
If the essence of the atonement is substitution, at least two important inferences follow, the first theological and the second personal. The theological inference is that it is impossible to hold the historic doctrine of the cross without holding the historic doctrine of Jesus Christ as the one and only God-man and Mediator. As we have seen, neither Christ alone as man nor the Father alone as God could be our substitute. Only God in Christ, God the Father’s own and only Son made man, could take our place. At the root of every caricature of the cross there lies a distorted Christology. The person and work of Christ belong together. If he was not who the apostles say he was, then he could not have done what they say he did. The incarnation is indispensable to the atonement. In particular, it is essential to affirm that the love, the holiness and the will of the Father are identical with the love, the holiness and the will of the Son. God was in Christ reconciling the world to himself.
Perhaps no twentieth-century theologian has seen this more clearly, or expressed it more vigorously, than Karl Barth. Christology, he insisted, is the key to the doctrine of reconciliation. And Christology means confessing Jesus Christ the Mediator, he repeated several times, as “very God, very man, and very God-man.” There are thus “three Christological aspects” or “three perspectives” for understanding the atonement. The first is that “in Jesus Christ we have to do with very God. The reconciliation of man with God takes place as God himself actively intervenes.” The second is that “in Jesus Christ we have to do with a a true man. . . He is altogether man, just as he is altogether God. . . . That is how he is the reconciler between God and man.” The third is that, although very God and very man, “Jesus Christ himself is one. He is the God-man.” Only when this biblical account of Jesus is affirmed can the uniqueness of his atoning sacrifice be understood. The initiative lay with “the eternal God himself, who has given himself in his Son to be man, and as man to take upon himself this human passion. . . It is the Judge who in this passion takes the place of those who ought to be judged, who in this passion allows himself to be judged in their place.” “The passion of Jesus Christ is the judgment of God, in which the judge himself was the judged.”
The second inference is personal. The doctrine of substitution affirms not only a fact (God in Christ substituted himself for us) but its necessity (there was no other way by which God’s holy love could be satisfied and rebellious human beings could be saved). Therefore, as we stand before the cross, we begin to gain a clear view of God and of ourselves, especially in relation to each other. Instead of inflicting on us the judgment we deserved, God in Christ endured it in our place. Hell is the only alternative. This is the “scandal,” the stumbling block, of the cross. For our proud hearts rebel against it. We cannot bear to acknowledge either the seriousness of our sin and guilt or our utter indebtedness to the cross. Surely, we say, there must be something we can do, or at least contribute, in order to make amends? If not, we often give the impression that we would rather suffer our own punishment than the humiliation of seeing God through Christ bear it in our place (pages 159-160).
— The Cross of Christ