Long before Machen wrote Christianity and Liberalism, he studied in Germany under the Ritschlian, Wilhelm Hermann.
Machen wrote home about the experience:
The first time that I heard Herrmann may almost be described as an epoch in my life. Such an overpowering personality I think I almost never before encountered—overpowering in the sincerity of religious devotion . . . .
My chief feeling with reference to him is already one of the deepest reverence . . . . I have been thrown all into confusion by what he says—so much deeper is his devotion to Christ than anything I have known in myself in the past few years . . . . Hermann affirms very little of that which I have been accustomed to regard as essential to Christianity, yet there is no doubt in my mind that he is a Christian, and a Christian of a peculiarly earnest type.
cited in John Piper, Contending for Our All: Defending Truth and Treasuring Christ in the Lives of Athanasius, John Owen, and J. Gresham Machen (Crossway, 2006), 123.
This reveals the fallacy of equating Christianity with piety apart from doctrine. It was an error the young Machen almost succumbed to and an error about which the older Machen tried to warn the church.