Rationalism is a complete denial of the supernatural. It's a belief system where Plato or Aristotle are the worshiped gods.
Relativism believes that truth is relative. This is where culture rules the day, and college ethics professors become the thought rulers for young adults.
Post-Modernism says nothing is absolute, and that authority is subject to change.
Then God steps in and says...
Isaiah 55:8-9 For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD. For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.