Photius of Constantinople

St. Photius the Great was Patriarch of Constantinople and one of the most learned men of the Byzantine world: his Bibliotheca (review of 280 books) and Lexicon are treasures of classical learning. His Mystagogy of the Holy Spirit is the foundational Orthodox theological refutation of the Filioque — arguing both that the Filioque is theologically erroneous (introducing a second principle into the Trinity) and that no portion of the Church may alter the Creed unilaterally. He is venerated as a saint and confessor.