I'm re-reading Christianity and Liberalism and finding it astonishingly relevant to the church today -- especially the Presbyterian branch. Some of the secondary issues and nuances have changed, but the fundamental conflict continues to revolve around differing views of Christ, the authority of Scripture, and the nature of salvation. The sickness of theological liberalism that Machen saw as diametrically opposed to Christianity results in a variety of symptoms.
Part of what Machen was responding to was an attempt to "rescue" Christianity from so much emphasis on doctrine, and to rehabilitate Jesus as the founder of a non-doctrinal religion that was later hijacked by the apostles and church fathers with their creeds and councils. The battle cry of the modernists was "Christianity is a life not a doctrine." Read Machen for yourself, but I think he succeeded in demonstrating that any such effort doesn't fly, and that the pitting of deeds against creeds results in something other than the New Testament gospel. In a sad irony it removes the fuel for changed lives.
Even if you get rid of John, that most doctrinal of the Gospels, and limit yourself to only those statements of Jesus that even the most critical scholars accept as authentic, one is forced to conclude that Jesus was more than a teacher of timeless moral truths. His Messianic consciousness is everywhere apparent. He proclaimed the coming of the Kingdom of God and began to explain to his disciples what that meant. It was left to the Apostles, guided by the Holy Spirit, to explain the full meaning of Christ's life, death and resurrection.
Here's Machen in his own words from Chapter 2 of Christianity and Liberalism:
From the beginning, the Christian gospel, as indeed the name "gospel" or "good news" implies, consisted in an account of something that had happened. And from the beginning, the meaning of the happening was set forth; and when the meaning of the happening was set forth then there was Christian doctrine. "Christ died"—that is history; "Christ died for our sins"—that is doctrine. Without these two elements, joined in an absolutely indissoluble union, there is no Christianity. (p. 27)
"Suffered under Pontius Pilate, was crucified, dead and buried"—that is history. "He loved me and gave Himself for me"—that is doctrine. Such was the Christianity of the primitive Church. (p. 29)
Jesus was certainly not a mere enunciator of permanent truths, like the modern liberal preacher; on the contrary He was conscious of standing at the turning-point of the ages, when what had never been was now to come to be. (pp. 31-2)