Why docetism fails

The humanity of Christ is also essential to God's act of reconciliation, for the actuality of atonement is grounded upon the fact that in actual human nature it is God himself acting on our behalf. Thus any docetic view of the humanity of Christ would mean that God only appears to act within our human existence, or that his acts are only of tangential significance, that they do not really strike into the roots of our existence and condition, and have no relevance to our need. Atonement is real and actual only if and as the mediator acts fully from the side of man as man, as well as from the side of God as God. If the humanity of Christ is imperfect, atonement is imperfect, and we would then still be in our sins. If Jesus Christ is really and truly man, then his death for sin is an act of God himself in human nature, and not just an external act upon human nature. But if atonement is to fulfil its object, it must be not only act of God upon man, but act of man in response to God, man's sacrifice, man's oblation, satisfaction by man for sin before God. Apart from the human obedience and human life and death of Christ, apart from his human sacrifice, we have nothing at all to offer to God, nothing with which we can stand before God, but our sin and guilt. But here in the full humanity of Jesus, as it is joined eternally to his deity in incarnation and atonement, man's destiny as man is actually assured and restored to its place in God from which it has fallen; man's wrong has been set aside in and with the judgement accomplished upon the humanity of Christ, and now in his humanity our new right humanity has been established before God.—T. F. Torrance, Incarnation: The Person and Life of Christ, 186