Johan Kemper (1670–1716), formerly Moshe ben Aharon Ha-Kohen of Kraków or Moses Aaron, baptized Johann Christian Jacob; was a Polish Sabbatean Jew who converted from Judaism to Lutheran Christianity. His conversion was motivated by his studies in Kabbalah and his disappointment following the failure of a prophecy spread by the Polish Sabbatean prophet Zadok of Grodno, which predicted that Sabbatai Zevi would return in the year 1695/6. It is unclear whether he continued to observe Jewish practices after his conversion.