This is a warning to the people living in Jerusalem to repent. "Cleanse your heart" is a metaphor for removing evil from their lives. Alternate translation: "People of Jerusalem, stop being evil" or "People of Jerusalem, stop living wickedly" (See: [[rc://en/ta/man/translate/figs-metaphor]])
This question is used to rebuke the people for always planning how to sin. Alternate translation: "Your deepest thoughts are about how to sin!" or "You are always thinking about how to sin!" (See: [[rc://en/ta/man/translate/figs-rquestion]])