Christianity and Liberalism by J. Gresham Machen is one of my all time favorite books. It is simple but very insightful. It gets to the root of the matter regarding the divide between liberal Christianity and biblical Christianity (we are talking about theological liberalism here and not political liberalism). In fact, Machen demonstrates that so-called “liberal Christianity” is a different religion altogether, for it has abandoned the core tenants of the Christian faith as reveled in Holy Scripture. The book was first published in 1923. Machen sounds prophetic at times as we consider how the liberal tradition has progressed over the past 100 years. It is also interesting to explore Machen’s relationship to the fundamentalist moment and his ultimate rejection of that movement in favor of a robust confessional tradition.
The book is available in print but you can also find PDF copies online for free.
Here is an interview of Darryl G. Hart concerning the book: Machen’s Christianity and Liberalism
You can find an audio recording of the book here: Reformed Forum, Theology Simply Profound