Here "darkened in their understanding" is a metaphor that means not to be able to understand the truth. And "alienated from the life of God" is a metaphor that means to not have the life that God gives. Alternate translation: "They do not understand what is true, and they do not have the life that God gives to people" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "hearts" is a metonym 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]])