"It is not surprising then that it [Liberalism] differs from Christianity in its account of the gospel itself; it is not surprising that it presents an entirely different account of the way of salvation. Liberalism finds salvation (so far as it is willing to speak at all of “salvation”) in man; Christianity finds it in an act of God.
"The difference with regard to the way of salvation concerns, in the first place, the basis of salvation in the redeeming work of Christ. According to Christian belief, Jesus is our Saviour, not by virtue of what He said, not even by virtue of what He was, but by what He did. He is our Saviour, not because He has inspired us to live the same kind of life that He lived, but because He took upon Himself the dreadful guilt of our sins and bore it instead of us on the cross. Such is the Christian conception of the Cross of Christ. It is ridiculed as being a “subtle theory of the atonement.” In reality, it is the plain teaching of the word of God; we know absolutely nothing about an atonement that is not a vicarious atonement, for that is the only atonement of which the New Testament speaks. And this Bible doctrine is not intricate or subtle. On the contrary, though it involves mysteries, it is itself so simple that a child can understand it. ”We deserved eternal death, but the Lord Jesus, because He loved us, died instead of us on the cross” - surely there is nothing so very intricate about that. It is not the Bible doctrine of the atonement which is difficult to understand - what are really incomprehensible are the elaborate modern efforts to get rid of the Bible doctrine in the interests of human pride.
"Modern liberal preachers do indeed sometimes speak of the “atonement.” But they speak of it just as seldom as they possibly can, and one can see plainly that their hears are elsewhere than at the foot of the Cross. Indeed, at this point, as at many others, one has the feeling that traditional language is being strained to become the expression of totally alien ideas. And when the traditional phraseology has been stripped away, the essence of the modern conception of the death of Christ, though that conception appears in many forms, is fairly plain. The essence of it is that the death of Christ had an effect not upon God but only upon man. Sometimes the effect upon man is conceived of in a very simple way, Christ’s death being regarded merely as an example of self-sacrifice for us to emulate. The uniqueness of this particular example, then, can be found only in the fact that Christian sentiment, gathering around it, has made it a convenient symbol for all self-sacrifice; it puts in concrete form what would otherwise have to be expressed in colder general terms."
J. Gresham Machen from Christianity And Liberalism