In Christianity & Liberalism, J. Gresham Machen writes the following on the atonement:
The atoning death of Christ, and that alone, has presented sinners as righteous in God’s sight; the Lord Jesus has paid the full penalty of their sins, and clothed them with His perfect righteousness before the judgment seat of God. But Christ has done for Christians even far more than that. He has given to them not only a new and right relation to God, but a new life in God’s presence for evermore. He has saved them from the power as well as from the guilt of sin. The New Testament does not end with the death of Christ; it does not end with the triumphant words of Jesus on the Cross, “It is finished.” The death was followed by the resurrection, and the resurrection like the death was for our sakes. Jesus rose from the dead into a new life of glory and power, and into that life He brings those for whom He died. The Christian, on the basis of Christ’s redeeming work, not only has died unto sin, but also lives unto God. ...
But how is the redeeming work of Christ applied to the individual Christian man? The answer of the New Testament is plain. According to the New Testament the work of Christ is applied to the individual Christian man by the Holy Spirit. And this work of the Holy Spirit is part of the creative work of God. It is not accomplished by the ordinary use of means; it is not accomplished merely by using the good that is already in man. On the contrary, it is something new. It is not an influence upon the life, but the beginning of a new life; it is not development of what we had already, but a new birth. At the very center of Christianity are the words, “Ye must be born again.”
These words are despised today. They involve supernaturalism, and the modern man is opposed to supernaturalism in the experience of the individual as much as in the realm of history. A cardinal doctrine of modern liberalism is that the world’s evil may be overcome by the world’s good; no help is thought to be needed from outside the world.