The following is from About.com/Christianity:
Postmodernism From Jack Zavada
Definition: Postmodernism is a philosophy that says absolute truth does not exist.
Postmodernism supporters deny long-held beliefs and conventions and maintain that all viewpoints are equally valid.
In today's society, postmodernism has led to relativism, the idea that all truth is relative. That means what is right for one group is not necessarily right or true for everyone. The most obvious example is sexual morality. Christianity teaches that sex outside marriage is wrong. Postmodernism would claim that such a view might pertain to Christians but not to those who don't follow Jesus Christ; therefore, sexual morality has become much more permissive in our society in recent decades. Taken to extremes, postmodernism argues that what society says is illegal, such as drug use or stealing, is not necessarily wrong for the individual.
Postmodernism's rejection of absolute truth causes many people to reject the Bible. Christians believe God is the source of absolute truth. In fact, Jesus Christ proclaimed himself to be the Truth: "I am the way and the truth and the life. No one comes to the Father except through me." (John 14:6, NIV).
Not only do postmodernists deny Christ's claim to be the truth, but they also dismiss his statement that he is the only way to heaven. Today Christianity is ridiculed as arrogant or intolerant by those who say there are "many paths to heaven." This view that all religions are equally valid is called pluralism.
In postmodernism, all religion, including Christianity, is reduced to the level of opinion. Christianity asserts that it is unique and that it does matter what we believe. Sin exists, sin has consequences, and anyone ignoring those truths has to face those consequences, Christians say.
For more on this topic see: "What are the dangers of postmodernism?"
Relativism / Postmodernism