Jesus uses a question to mildly rebuke them. Jesus was telling them not to doubt that he was alive. The word "heart" is a metonym for a persons' mind. Alternate translation: "Do not doubt in your minds!" or "Stop doubting!" (See: [[rc://en/ta/man/jit/figs-rquestion]] and [[rc://en/ta/man/jit/figs-metonymy]])