Celestine I succeeded Boniface I in 422. His ten-year pontificate is remembered for two consequential decisions: he commissioned St. Cyril of Alexandria to act in his name at the Council of Ephesus (431), where Nestorius was condemned and the title Theotokos defined; and he consecrated Patrick — a Britain-born missionary — and sent him to Ireland in 432, establishing the see of Armagh.
He also sent Bishop Palladius to the Christians already in Ireland, opposed the rise of semi-Pelagianism in Gaul, and corresponded extensively with the bishops of Africa on the condemnation of Pelagius. The famous principle 'lex orandi, lex credendi' ('the law of prayer is the law of belief') derives from his pontificate.