Sixtus of Siena or Sixtus Senensis (1520 - 1569), a converted Jew, followed a Franciscan course of study and became a Roman Catholic theologian. Though he was convicted of heresy he was saved by a Dominican inquisitor, the future Pope Pius V, who repealed the condemnation when Sixtus recanted and pledged to transfer to the Dominican Order instead. He is considered one of the two outstanding Dominican scholars of his generation. He had as a master Lancelotto Politi, some of whose writings he later publicly criticised.
Sixtus coined the term deuterocanonical to describe certain books of the Old Testament that had not been accepted as canonical but which appeared in the Septuagint, and the definer for the Roman Catholics of the terms protocanonical and the ancient term apocryphal.
His work Bibliotheca sancta ex præcipuis Catholicæ Ecclesiæ auctoribus collecta (Venice 1566) treats the sacred writers and their works, the best manner of translating and explaining Holy Writ, and gives a copious list of Biblical interpreters, in eight books. It was the first of the genre of encyclopedic teaching repertories of dogma and Church tradition issued in the wake of the Council of Trent.
- John Warwick Montgomery, 1963. "Sixtus of Siena and Roman Catholic Biblical Scholarship", Archiv für Reformationsgeschichte 54 p 214ff.
- Fernando Dominguez, "Sixtus von Siena", in: Lexikon für Theologie und Kirche 3 IX, 647.
- http://www.ofmconvmalta.org/interconv/culture/schol.htm "Franciscan scholars": Sixtus of Siena]