Italian convert to Christianity and anti-Talmudic agitator; born at Sienna (whence his name) in 1520; died in 1569. After his conversion Sixtus entered the Franciscan order, but soon after, being charged with heterodoxy, he was sentenced to death at the stake. Cardinal Michele Ghislieri, later Pope Pius V., recognizing in Sixtus one who might be useful to him, rescued him from death and helped him to enter the Dominican order. At the command of Paul IV., Sixtus and another convert, Philip Moro, traveled about the Papal States preaching in the synagogues and inciting the mob against the Jews wherever the latter resisted the exhortations to embrace Christianity. In April, 1559, Sixtus, with another Dominican monk, went to Cremona to burn the Talmud, declaring that it contained only anti-Christian writings. Yet he spared the Zohar and restrained the Spanish soldiers from destroying it with the Talmud, hoping that the Jews might be induced by means of this cabalistic work to embrace Christianity. Besides homilies and mathematical writings, Sixtus was the author of the "Bibliotheca Sancta"(Venice, 1566), a Latin work in eight books, treating of the divisions and authority of the Bible; it contains an alphabetical index and an alphabetical list of rabbinical interpreters of the Bible.
- Grätz, Gesch. 3d ed., ix. 358-359, 362;
- Nouvelle Biographie Générale;
- Wolf, Bibl. Hebr. i., No., 1751.