Here "not doubt" is a double negative meaning "truly believe" and "heart" is a metonym for "inner thoughts and intentions." Jesus says it both ways for emphasis. AT: "if he truly believes in his heart" or "if he does not doubt but believes" (See: [[rc://en/ta/man/translate/figs-doublenegatives]] and [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-metonymy]])