In Romans 1, Paul writes of the sinfulness of people who reject God and choose sin. The people who received the letter would have been nodding their approval at Paul’s harsh words about “those people” and the godless way they live.
In Romans 2, Paul drops a bomb by holding a mirror up so his readers (who claimed to choose God and reject sin) could see themselves. “You may think you can condemn such people, but you are just as bad, and you have no excuse! When you say they are wicked and should be punished, you are condemning yourself, for you who judge others do these very same things” (Romans 2:1-1).
The people who rejected God were aware of their sinfulness. Those who claimed to be God’s people thought they were righteous, but they were actually self-righteous, and not self-aware.
The issue is that this was not just an issue in Rome 2,000 years ago. It is still an issue for people who claim God today.