The basic premise of the book, written at the beginning of the 20th century when the "mainline" denominations were starting to rot from the inside out, is that Christianity is not divided into conservatism and liberalism (or progressivism in today's parlance where liberal is a dirty word). Rather the divide is between Christianity and liberalism, and Machen is in no way subtle when he declares liberalism to be by definition not Christian and in fact a completely separate faith system. Here is Machen at his un-nuanced best:
But one thing is perfectly plain – whether or not liberals are Christians, it is at any rate perfectly clear that liberalism is not Christianity. And that being the case, it is highly undesirable that liberalism and Christianity should continue to be propagated within the bounds of the same organization. A separation between the two parties in the Church is the crying need of the hour. J. Gresham Machen (0100-12-31 17:00:00-07:00). Christianity & Liberalism (Kindle Locations 2201-2204)
Or as I like to put it, progressives might be Christians but if so it is in spite of, not because of, their progressive notions. Talk of separation over doctrine makes many religious people run for their security blanket but being unequally yoked is always destructive to the witness of the church.
Machen hits on the main topics that have led to the downfall and slow death of "mainline" Protestantism, topics like the denial of what the Bible teaches on issues like the nature of Christ, the atonement and the church and of course the main problem, a gross misunderstanding of the nature of the Bible itself. What continually stood out for me was that the same problems he pointed to almost a century ago are still infecting groups today. Not content with destroying some of the denominations that were early adopters of a functional universalism, evolution and a gender-less church, the same destructive ideas are infecting groups like the Mennonite Church-USA, and like other denominations the message is that the MC-USA must capitulate and adopt these errors or risk dying, even though the evidence suggests just the opposite.
The language used Christianity & Liberalism often sounds jarring to our tender ears, steeped in contemporary political correctness as they are. I would say that we need more talk like this, unashamed, unapologetically blunt and devoid of nuance. We as the church have been nuancing ourselves to death for a century and the result is a post-apocalyptic landscape of dying local congregations and toothless church leaders who are terrified of offending the people putting the money in the offering plate and emasculated by the thought of being mocked by the haughty learned in religious circles for clinging to a primitive religious system.
Read Christianity & Liberalism today, the events Machen foretold are happening all around us and there is no time to waste.