Changed the Glory of God

In these verses, Paul makes very clear that all men stand guilty before God. That God has declared His nature to them through Creation, so that everyone understands that there is a God that they will have to answer to. The response of natural man is not to repent, but deceive himself into believing that he does not know what all of Creation testifies to. Because they want to exalt themselves, they refuse to recognize the thankfulness that is due to God and instead become fools and blind to reality. In that blindness, they then go to worshipping the creature rather than the Creator. That worship of the creature makes them slaves of their flesh rather than reasoning creatures made in the image of God.