The word "liberal" is useless because there is no consensus on what it means. Similarly useless are the words "left" and "right". "Marxist" is still useful because nobody can deny that it denotes adherence to the ideas of Karl Marx. Similarly, even though people TRY to erode its meaning, "Christian" must ultimately mean adherence to the philosophies espoused by Christ. So for example anyone who identifies as "Christian" but makes no attempt to follow the principles he espoused, is simply lying.