Just because the knowledge of God disappears from a nation does not mean that worship stops. In Romans 1, we see that man turns from worshipping the Creator to worshipping the creature. People will fashion an idol and continue to worship. This is a scary thing to consider. It is evidenced in the Old Testament and warned against in the New Testament.

Today we still bow down before false gods.

If you forget the one true God, you won't become an atheist. You will simply be worshipping something else.

When the knowledge of God disappears in a nation, worship doesn't stop. So in Romans 1, we see that they turn from worshiping the Creator to worshiping the creature, which is a scary thing to consider, that people will continue to worship and they will fashion an idol. We see that all throughout the Old Testament. Sometimes we forget that we do the same now. You see in the New Testament warnings about idolatry and that in our present day people really are bowing down to False gods so if you forget the one true God, it's not like anyone really becomes an atheist They will be worshiping something it just won't be the one true God and when that one true God is not worshipped, well then all sorts of terrible things result for a people when that happens.