In 1346 he wrote his Sefer ha-Mora, a refutation of the philosophical views on the Creation. Believing that he had placed Rabbinism on a sure foundation, Shemariah undertook, in 1352, a journey to Castile and Andalusia, in order to convert the Karaites. He is said to have pretended to be the Jewish Messiah, and was reviled to such an extent that the government arrested him.