Orthodox Saints of the Pre-Schism See of Rome
AMAETHLU (MAETHLU), a sixth century Welsh saint for whom the church and village of Llanfaethlu in Anglesey is named. Nothing further is known about his life.
DIONYSIUS, twenty-fifth Pope of Rome, assuming the throne in 259 following a one year interregnum caused by the Valerian Persecution (257–260). As Pope, St. Dionysius opposed Sabellius and condemned Paul of Samosata’s heretical writings. St. Dionysius reposed in 268, the first Pope of Rome not to have been martyred.
MARINUS, the son of an Imperial Roman Senator, and possibly a senator himself. Tortured and martyred for being a Christian in 283.
TATHAI (TATHAN, TATHAEUS, ATHAEUS), an Irish monk or hermit who, in the late fifth or early sixth century lived in an area that was called Llantathan in Glamorgan, Wales, after him. He is said to have founded a monastery there, which at the time, was quite well known. Some sources state that St. Tathai founded another monastic school in Gwent where St. Cadoc (24th January) is said to have been a student.
THEODORE the SACRIST, (Sixth Century), a sacristan at St. Peter’s Basilica in Rome. The limited information on his life comes from the writings of his contemporary of St. Gregory the Dialogist (3rd September).
ZOSIMUS, elected forty-first Pope of Rome in March 417, St. Zosimus spent a great deal of his papacy advancing the cause of Papal Supremacy. He fought against Pelagianism, even engaging Pelagius himself. St. Zosimus reposed in December 418.
Prior to the Schism the Patriarchate of Rome was Orthodox, and fully in communion with the Orthodox Church. As Saint John of Shanghai and San Francisco +1966 said “The West was Orthodox for a thousand years, and her venerable Liturgy is far older than any of her heresies”.
Details of British Saints excerpted from Orthodox Saints of the British Isles.
Details of continental saints from these sources.
In many cases there are several spelling versions of the names of saints from the British Isles. I use the Oxford Dictionary of National Biography version as the primary version with the more prevalent version in parenthesis e.g. Ceadda (Chad) of Lichfield.