J. Gresham Machen states it as clearly as it can be said:
It is no wonder, then, that liberalism is totally different from Christianity, for the foundation is different. Christianity is founded upon the Bible. It bases upon the Bible both its thinking and its life. Liberalism on the other hand is founded upon the shifting emotions of sinful men (Christianity and Liberalism, new edition, Eerdmans, 2009, xv).