It is supposedly a less dogmatic position than that of atheism, holding that one can neither prove nor disprove the existence of God. Charles Darwin, as well as Huxley, professed to be an agnostic, although both seemed to waiver back and forth between agnosticism and atheism in their writings and correspondence. At its core is the loss of belief in any kind of Truth, and therefore the loss of belief that Right and Wrong exist. To the postmoderns everything is relative. What may be true or right for one person may not be true or right for the next.
Ethics is a matter of taste, and what's right is merely a pragmatic question of what works.
This attitude even is starting to affect the teaching of evolutionary biology. It seems that these postmodern liberals not only reject Christianity, but science as well, especially when its findings and theories are presented as objective truth. We can sum them up in one sentence. Anybody who claims to have objective knowledge about anything is trying to control and dominate the rest of us. And though all fields of science are suspect, what most left-wing anxiety centers on is biology.