Johan Christian Jacob Kemper (1670-1716), formerly Moshe ben Aharon of Kraków, 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 Tzadok of Gordno, which predicted that Sabbati Zevi would return in the year 1695/6, It is unclear whether he continued to observe Jewish practices after his conversion.