An able speaker, and possessed of some knowledge of rabbinical literature, he persuaded King Henry II of Castile that he could convince the Jews of the truth of Christianity if they were obliged to listen to him and to answer his questions.
In company with another Jewish convert, John traveled throughout the Castilian provinces and lectured and debated in the synagogues, but with lack of success.
At Burgos he summoned Moses ha-Kohen of Tordesillas to a religious controversy in the presence of Archbishop Gomez of Toledo.
John endeavored to demonstrate from the Bible the Messianic claims and the divinity of Jesus, and the truth of the dogma of the Trinity and of other Christian doctrines.
Moses ha-Kohen refuted arguments of this kind, and the controversy was broken off in the middle of the fourth sitting.