I recently learned about a very important (an unknown) Greek theologian. His name is Leo Allatius. He was born in 1586 and died in 1669. He graduated from the College of Saint Athanasius in Rome where he later taught Greek and theology. Allatius is remembered for providing the theological justification for the preservation of rites and traditions of the Eastern Churches in communion with Rome.
Allatius's knowledge in these matters established him as an "ecumenical theologian" between the Eastern and Western Churches. Allatius stressed that Greek dogma was identical to Roman dogma and that the schism was merely juridical. He defended this 1648 thesis in his De Ecclesiae occidentalis atque orientalis perpetua consensione ("The Western and Eastern Churches in perpetual Agreement"). 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.
Three years later in 1651 Allatius published his edition of the works of George Acropolites. George Acropolites had served as the Byzantine Emperor's ambassador in the 13th century. George was notable because he had been a defender of the Pope's juridical supremacy over the Eastern Churches. These works were obviously a treasure chest for Eastern Catholic apologetics.
Two "fun facts" about Leo Allatius. He was a fan of the opera and catalogued all known operatic dramas. Music historians are therefore very grateful to him. He was also a physician and wrote about the pheneomenon of vampires. Go figure.