(31 July 432–19 Aug. 440)
A Roman by birth, son of Xystus, he had earlier had the reputation of being sympathetic to Pelagius, and had been claimed by Pelagians as an ally; but when Zosimus published (418) his Tractoria, he publicly anathematized Pelagianism and made his rejection of it clear to its archadversary, Augustine. Nothing is known of his activities during the reign of Boniface I, but references in his early letters to Celestine I's correspondence with the east after the council of Ephesus (431) suggest that he had a hand in drafting it.
Sixtus continued Celestine I's policies, working hard, in collaboration with Emperor Theodosius II (408–50), to heal the breach which had opened at Ephesus between Cyril of Alexandria and John of Antioch (d. 441), leading proponent of the moderate Antiochene Christology. Avoiding mention of Cyril's anathemas, he insisted that John and others sharing his views only needed to accept the decisions of Ephesus and disavow Nestorius to be restored to communion. The reconciliation reached in spring 433 on the basis of the Symbol of Union, drafted by the Antiochenes but accepted by Cyril, gave him great satisfaction; he attributed the success to the Apostle Peter, guarantor of true faith and present in himself. The excellent relations Rome now enjoyed with the east were temporarily clouded when Proclus, the new bishop of Constantinople (434–46/7), initiated moves in 434 to detach east Illyricum (south-east Balkan peninsula) from its traditional ecclesiastical subjection to Rome. He had to warn the Illyrian bishops, who were showing signs of insubordination, to pay no attention to oriental synods, and to remind them that the bishop of Thessalonica was still his vicar in east Illyricum. At the same time he requested (Dec. 437) Proclus not to receive bishops from Illyricum who failed to produce letters of credence from his vicar, Anastasius of Thessalonica. To conciliate Proclus, he informed him that a bishop of Smyrna who had been sentenced at Constantinople had appealed to Rome, but that he had simply endorsed his sentence. In 439, stiffened by his deacon Leo, he resisted the pleas of the Pelagian leader Julian of Eclanum (d. 454), who had been deposed and exiled in 418, to be allowed to return to his see in Apulia.
Sixtus founded the earliest recorded monastery in Rome at S. Sebastiano on the Appian Way. Helped by funds provided by the imperial family, he also carried out a more noteworthy building programme than any one of his predecessors, one of his motives being to make good the destruction wrought by the Visigoths in 410. He deliberately used two of his most remarkable works, his new octagonal baptistery at the Lateran and his reconstruction of the Liberian basilica as Sta Maria Maggiore, to advertise the dogmatic achievements of his age; the inscriptions in the former extolled divine grace and the theology of baptism, thus underlining the defeat of Pelagianism, and the mosaics of the latter celebrated the church's triumph over Nestorianism. His cult was late in developing, his name first appearing in the 9th-century Martyrology of Ado. Feast 19 Aug.