By “Christian theology” I mean, more specifically, biblical theology. That is, what the Bible says about God and how He does things. If you’re looking for erroneous things that some Christians might believe, you’ll find some in the False Teaching section.
Postmodernism is the idea is that reality is subjective...that there is no absolute truth, and that reality differs from person to person. The phrase "what's true for you might not be true for me" typifies a postmodern point of view.