Someone who accepts (or claims to accept) at least some of the basic truths of Christianity, but who does not regard these truths as the most important truths in the world. In other words, a liberal Christian is someone who accepts (or claims to accept) at least some propositions that are essential to Christianity, but whose basic worldview is not Christian. The liberal Christian puts something other than Christianity first, and Christianity second—at best.
The differences between liberal and orthodox Christians become obvious when some secular practice or belief cherished by the Christian or his peers contradicts something taught by his faith. In such cases, the orthodox Christian has only one morally and rationally tenable option: He must reject that belief or practice in favor of orthodoxy. The liberal may do this, but then again he may not. If said practice or belief is more important to him than the truths of Christianity, he will gladly abandon the truths of Christianity. If naturalism contradicts Biblical inerrancy, he may drop the inerrancy; if utilitarianism contradicts the Ten Commandments, he may drop the Ten Commandments; and if innovation for its own sake is more important to him than two millennia of Church tradition, he will drop Tradition.
It is clear that liberal Christianity is an absurd position, since it is also clear that Christianity, if it is true at all, is the most important truth in the universe. It is also clear that despite what the name suggests, liberal Christians need not necessarily be very liberal. They certainly can be, and in the modern West, they almost always are. But on my definition, the Protestant Reich Church was a paragon of liberal Christianity, since it put National Socialism first, with Christianity at a distant second. This may seem counterintuitive, until you learn that it was generally liberal ministers who most readily bent their knee to the Nazis.
Does this mean that this “pro-Western Christianity” we’ve been hearing so much about is a form of liberal Christianity? Not at all; a pro-Western attitude is consistent with—and even entailed by—Christianity. As someone once said, Europe is the Church, and the Church is Europe. “Pro-Western Christianity” is as much of a pleonasm as is “theistic Christianity” or “Jesus-based Christianity.” The fact that we need to use the expression at all does not show that our beliefs are contradictory, but that the beliefs of mainstream Christians often are as contradictory as they are un-Christian.