Not to be confused with Pope Sixtus V who later was involved in the Vulgate edition that was quickly supplanted by the Clementine, with Bellarmine having a major role in the shift.Sixtus (1520-1569)
Sixtus of Siena (or Sixtus Senensis) (1520–1569) was a Jew who converted to Roman Catholicism, 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 Though he was convicted of heresy he was saved by a Dominican inquisitor, the future Pope Pius V, who repealed the condemnation when Sixtus recanted and pledged to transfer to the Dominican Order instead.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 most outstanding Dominican scholars of his generation. He had as a master Lancelotto Politi, some of whose writings he later publicly criticised.
 The other being Santes Pagnino (Benedict Ashley, History of the Dominican Order).
Pagninus (1470-1541) was involved with the 1495 Vulgate edition that is mentioned by Porson in the question of Walafrid Strabo and the Prologue. Grantley mentions his 1527 Vulgate edition from Lyon as having the heavenly witnesses verse. So far, no other heavenly witnesses references. He is also helpful on the right side of the Jehovah question.
Looks like an interesting paper:Introduction to the Canonical Books of the Old Testament, Tr. by G.H. Box (1907)
Carl Heinrich Cornill
The "Bibliotheca Sancta” of Sixtus Senensis (f 1599), which first appeared in 1566, and was repeatedly reprinted and reissued down to the eighteenth century, can be described as an attempt in the direction of a history of Biblical literature, even though the larger part of its pages is still occupied with hermeneutical matter and the history of the interpretation.
Gesenius compliments his work as an introduction to the Bible.Internationale Zeitschrift zur Erforschung der Reformation und ihrer Weltwirkungen - An international journal concerned with the history of the Reformation and its significance in world affairs
Sixtus of Siena and Roman Catholic Biblical Scholarship in the Reformation Period (1963)
John Warwick Montgomery
For more on a summary of Biblical Interpretation luminaries, referencing Sixtus and others, we have an 1864 Cyclopaedia note by Karl August Credner:Essays and Dissertations in Biblical Literature: Containing Chiefly Translations of the Works of German Critics, Volume 1 (1820)
History of Introductions to the Scriptures, by William Gesenius; translated from the German, by Samuel II. Turner,
The kind of learning which I have been describing is, as has been remarked, the growth of the last century, and is indebted principally for its origin to the discussions of German Protestants on the various subjects connected with the Bible; and the name, as now usually applied, was first employed by J. G. Carpzov. A work in some respects similar to an introduction to the Bible was first given to the world by Augustin in his Doctrina Christiana, which, however, is rather hermeneutical advice in reading the Scriptures. This was followed in the sixth century by a production of Cassiodorus, who begins his directions for the study of theological literature with an account of the books of Scripture and their interpreters. In modem times Sixtus Sinensis first collected together the materials belonging to this subject in his Bibliolheca Sancta, § which remained an universally esteemed manual, until it was supplanted, at least among Protestants, by Walther's Officina Biblica, a pretty meagre production.||
In the heavenly witnesses debate, today Sixtus is mentioned mostly because of how Turretin references him in an appeal to Greek evidences. Turretin is mangled by contras, an area we look at elsewhere. A careful translation of the phrasing used by Sixtus will help in that controversy.
Sixtus is also referenced by Richard Simon in discussing the ending of Mark, with Simon criticizing some comments from Sixtus, who apparently wanted to limit the context of the comments of Jerome.Sixtus of Siena (d. 1569) A Jewish convert, his exegesis was too progressive for many of cautious temperament. However, his intuitive Inquisitor repealed the condemnation when he recanted and pledged transferral to the Dominicans. Thus was a Franciscan scholar delivered by his judge, the future St. Pius V (Dominican). Sixtus assumed charge of reviewing Hebrew texts, preserving a heritage that others were not yet able to appreciate. The mandate of Trent to codify the scriptures was greatly advanced through his formulation of the terms: protocanonical, deuterocanonical and apocryphal.
The main texts:Black, Crofton. “Eucherius of Lyon, Giovanni Pico della Mirandola and Sixtus of Siena: Early Christian Exegesis and Kabbalah in the Bibliotheca sancta (1566),” in Giovanni Pico e la cabbala, edited by Fabrizio Lelli (Firenze: Leo S. Olscbki Editore, 2014), pp. 231-258.
As referenced in:
The Study of Christian Cabala in English (2019)
Crofton Black and Don Karr are on Academia.edu
after 1069 pages comes two long Indexes.
Page reference from Grantley is p. 972 which is meant to answer the objections from p. 970:
Also we have:
p. 31 - Higinus is referenced, first here (in the verse context)
p. 472 - mentiones Targum Onkelos and other resources from his Jewish background, do they connect to the verse?
p. 903 - Cajetan and Amborse
p. 970 - heretical objections - this includes mentioning Jerome's Vulgate Prologue
The later editions may not maintain all the material, here is a bit from a 1742 edition:
footnote (Thomas Milante 1689-1749) Hyginus epistle , Cyprian's Unity of the Church and much more
https://books.google.com/books?id=f3zWFBHuEUsC&pg=PA32 - p. 41 in the 1566 edition
https://books.google.com/books?id=f3zWFBHuEUsC&pg=PA477 - Syriac edition
Line 5 has Iginus Papa
there are a few posts that might be used by Turretin, have to check his wording
in codicibus graecis ecclesia
in omnibus Graecis exemplaribus --> Turretin
all the Greek copies have it [habent tamen omnia Exemplaria Graeca], as Sixtus Senensis acknowledges: "they have been the words of never-doubted truth, and contained in all the Greek copies from the very times of the apostles" [et in omnibus Graecis exemplaribus ab ipsis Apostolorum temporibus lecta] (Bibliotheca sancta , 2:298).
This looks to be Volume 2, 1069 pages, plus indexes, no obvious heavenly witnesses references.
by Sisto (da Siena)