Sixtus III, also known as Xystus III, was born in Rome toward the end of the fourth century. Before his election he was already a leading figure among the Roman clergy and exercised influence under Popes Zosimus, Boniface I, and Celestine I. He corresponded with Saint Augustine of Hippo. Though some later writers linked him in youth with Pelagian circles, his exchange with Augustine and his subsequent conduct show firm adherence to orthodoxy; his conciliatory character later drew false accusations of sympathy with Pelagian or Nestorian error.
He was consecrated bishop of Rome on 31 July 432. As pope he approved the Acts of the Council of Ephesus (431), which affirmed Mary as Theotokos, Mother of God. He labored to restore peace between Cyril of Alexandria and John of Antioch; their reconciliation in 433 was sealed by the Formula of Union. He rejected the attempt of the Pelagian Julian of Eclanum to regain full communion with the Church. He defended the rights of the Apostolic See over Illyricum and confirmed the archbishop of Thessalonica as head of the Illyrian church against the ambitions of Proclus of Constantinople. In 437 he restored Bishop Brice of Tours to his see after unfounded charges. He is credited with nine (or eight attested) letters; certain ascetic treatises once ascribed to him are not authentic. A later accusation of crime attributed to the consul Bassus is regarded as a forgery.
His reign is closely associated with a renewal of sacred building in Rome. Santa Sabina on the Aventine was dedicated in his time. He rebuilt the old Basilica of Liberius as the Basilica of Saint Mary Major, the great Marian church whose dedication answered the teaching of Ephesus; tradition places its consecration on 5 August 434, and an inscription on the triumphal arch records his gift of new roofs to the Virgin Mary. He enlarged the Basilica of Saint Lawrence outside the Walls, founded an early monastery at Saint Sebastian on the Appian Way, and provided a new baptistery at the Lateran. From the emperor Valentinian III he obtained precious gifts for Saint Peter's and the Lateran basilica.
Sixtus III died in Rome on 18 (or 19) August 440 and was succeeded by Leo I. He was buried at the Basilica of Saint Lawrence outside the Walls. Veneration of him as a saint rests on ancient liturgical tradition and inclusion in martyrologies rather than a modern formal cause; he appears as a saint by the ninth century in the Martyrology of Ado. The Catholic Church and the Eastern Orthodox commemorate him; feast days associated with him include 18 August and the traditional Roman Martyrology date of 28 March.
Imitemur hodie sanctitatem huius Vitae, et Sixtus III intercedat pro nobis.
