What does Romans 1:23 mean?

Paul is describing how humanity becomes so desperately unrighteous and earns God's wrath against our sinfulness. So far, he has shown that in spite of the fact that God has made some things about Himself obvious in what He has made in creation, unrighteous human beings (Romans 3:23) refuse to see Him there (Romans 1:18–20). Having rejected Him as creator, why would we honor Him? What would we give Him thanks for providing for us through His creation? We wouldn't. Of course, this leaves us to ponder the universe as if there is no God. That leads us to all kind of worthless conclusions. Our thinking becomes futile, and our hearts becomes dark. Even worse, in the midst of our foolishness, we think we possess great wisdom. Our view of the world is upside down (Romans 1:21–22).

The next step down on this ladder of unrighteousness is that we begin to worship the creation instead of the Creator. We refuse to give God credit and instead we honor created things. That's not right. As Paul describes this idol worship, we trade the glory of the immortal God for images that look like mortal people and animals.

In other words, God has revealed Himself by what He has made as a glorious and eternal being. We should see this in the beauty of creation (Psalm 19:1). How could He who made that not be a glorious being Himself? Look at how long creation has gone on and on beyond the lifespan of men. How could He who made that not have an existence longer than everything that exists?

Humanity, however, having rejected the creator God, creates our own, lesser versions of the mortal things He has made, and we worship those. We worship our pale version of the creation instead of the creator of it all.