Christianity and Liberalism - J. Gresham Machen

First published in 1923 this little book has become a classic apologetic for biblical Christianity over against theological liberalism. From Chapter I:

...we shall be interested in showing that despite the liberal use of traditional phraseology modern liberalism not only is a different religion from Christianity but belongs in a totally different class of religions. But in showing that the liberal attempt at rescuing Christianity is false we are not showing that there is no way of rescuing Christianity at all; on the contrary, it may appear incidentally, even in the present little book, that it is not the Christianity of the New Testament which is in conflict with science, but the supposed Christianity of the modern liberal Church, and that the real city of God, and that city alone, has defences which are capable of warding off the assaults of modern unbelief. ... Here as in many other departments of life it appears that the things that are sometimes thought to be hardest to defend are also the things that are most worth defending.