Here "bring to light the hidden things of darkness" is a metaphor for making known to everyone things that were done in secret. Here "heart" is a metonym for people's thoughts and intentions. AT: "Like a light that shines on things in darkness, God will show what people have secretly done and what they secretly planned" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])