en_tn/rom/01/21.md

became foolish in their thoughts

"began to think foolish things"

their senseless hearts were darkened

Here "darkness" is a metaphor that represents the people's lack of understanding. Here "hearts" is a metonym for a person's mind or inner being. Alternate translation: "they became unable to understand what God wanted them to know" (See: rc://en/ta/man/jit/figs-metaphor and rc://en/ta/man/jit/figs-metonymy)