It’s easy when you are reading the first chapters of Romans to think ‘Ok, Paul, I get it. Jesus died for us and made us right with God. Can’t we move on from this?’
But he obviously thought it was really important for the Christians in Rome to realise just what God had done in and through Jesus.
So after the highpoints of the end of the 3rd chapter, and the exploration of faith with Abraham as an example, he takes us back to explain why we got in a mess in the first place, and how we can get out of it and stay, out of it. This is the gist of the sermon.
I hope it helps you, is encouraging and raises some further thoughts in your minds that you will run with.