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Leo Allatius, portrait in the Collegio Greco of Rome

Leo Allatius (Leone Allacci, circa 1586 - January 19, 1669) was an energetic Greek Catholic scholar and theologian.

Allatius was born in Chios around 1586, a distinctly Eastern Orthodox environment. A graduate of the Greek College of St. Athanasius in Rome, he spent his career in Rome where he found a patron in Pope Gregory XV. On Gregory's death in 1623, Allatius lost his principal patron, but with the support of influential churchmen, he continued his researches. Pope Alexander VII appointed him custodian of the Vatican Library in 1661, where he remained till his death. His cultural background, embracing the Greek and Roman worlds, afforded him a unique view of the age-old question of union to heal the Great Schism. Better than any western scholar of his day he knew the religious, historical and artistic traditions of the Orthodox world, struggling under Ottoman domination. More passionately than any other 17th century theologian, he believed that familiarity with these traditions would enable the two churches to bridge their theological and ecclesiastical divide.

Thus in 1651, when he published the first printed edition of the works of George Acropolita, the 13th century emissary of the Byzantine Emperor who acknowledged the supremacy of the Roman pontiff and thus had become something of a celebrity, at least in the West, the Latin essay that formed the preface to this volume, De Georgiis eorumque Scriptis gained fame itself as a learned plea for the commonalities between the two churches.

Allatius was a natural apologist for the Eastern communions in Eastern Europe, convinced as he was in himself that in the acts of union neither reasons of faith nor of doctrine were fundamental to the succession of the bishops, only a transfer of jurisdictions, and he seems really to have believed that the "Latin faith" and the "Greek faith" were identical and that under "Roman obedience" they could still be Orthodox. So he argued in his contribution to the mid-17th century Uniate pamphlet De Ecclesiae occidentalis atque orientalis perpetua consensione libri tres ("The Western and Eastern Churches in perpetual Agreement, in Three Books") (Cologne, 1648). Such notions led to the final stipulations that the Eastern Churches were not to be merged with the Catholic Church but would retain their own hierarchical independence and traditional rituals.

In 1623, when the Protestant Elector Palatine was supplanted by a Catholic one, thanks to the adroit political maneuvers of Pope Gregory XV and a grateful and successful Catholic candidate, Duke Maximilian I, Elector of Bavaria, presented the Pope with the Palatinate library of Heidelberg, containing about 3500 manuscripts, it was. Leo Allatius who was selected to transported the valuable collection to the Vatican Library. All but 39 of the Heidelberg manuscripts, which had been sent to Paris in 1797 and were returned to Heidelberg at the Peace of Paris in 1815, and a gift from Pope Pius VII of 852 others in 1816, remain in the Vatican Library to this day.

Allatius was trained as a physician. In 1645 he included the first methodical discussion of vampires, in De Graecorum hodie quirundam opinationibus ("On certain modern opinions among the Greeks"). In his later years he collected Greek and Syrian manuscripts to add to the late Pope Gregory XV's Eastern Library at the Vatican.

Of all his works, however, perhaps his most famous today is a minor essay De Praeputio Domini Nostri Jesu Christi Diatriba ("Discussion concerning the Prepuce of our Lord Jesus Christ"), which appears humorous to many modern readers. In this essay he speculated that the Holy Prepuce (Foreskin) may have ascended into Heaven at the same time as Jesus himself and might have become the rings of Saturn (then-recently observed).

His manuscripts (about 150 volumes) and his voluminous scholarly correspondence are in the library of the Oratorians in Rome.

Allatius died in Rome in 1669.

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