Man's Rebellion Against God Comes To Its Inevitable Fruition (1:21-25).
Paul now demonstrates how man's refusal to know God results in man's fall into gross sin. We have already been told about the ungodliness and unrighteousness of men in Romans 1:18. Paul now expands on that, dealing firstly with man's ungodliness as manifested by his turning to idols, with its inevitable consequences, in Romans 1:21-27. He will then move on to deal with man's unrighteousness as manifested by a list of gross sins (28-30).
One consequence of man's turning away from the true God is that men have to seek an alternative which will satisfy their inner cravings, which will fill ‘the God-shaped blank in every man's life'. And for long centuries they did this by associating the supernatural with human and animal forms. They saw these humans and animals as in some way a representation of the divine. Today we tend to do it by exalting celebrities and giving them a form of worship. In either case they lead on to the debasement of men and women.