They no longer think or reason clearly. Alternate translation: "They have darkened their thoughts" or "They are not able to understand" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])
This can be stated in active form. Alternate translation: "Because they do not know God, they cannot live the way God wants his people to live" or "They have cut themselves off from the life of God by their ignorance" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Here "hearts" is a metaphor for people's minds. The phrase "hardness of their hearts" is a metaphor that means "stubbornness." Alternate translation: "because they are stubborn" or "because they refuse to listen to God" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-metaphor]])