The virtuous life of the saint and his authority as a theologian won him the general esteem and love of the clergy and people. After the death of Saint Boniface (418-422), Saint Celestine was chosen to be the Bishop of Rome.
During this time, the heresy of Nestorius emerged. At a local Council in Rome in 430, Saint Celestine denounced this heresy and condemned Nestorius as a heretic. After the Council, Saint Celestine wrote a letter to Saint Cyril, Archbishop of Alexandria (January 18), stating that if Nestorius did not renounce his false teachings after ten days, then he should be deposed and excommunicated.
Saint Celestine also sent a series of letters to other churches, Constantinople and Antioch, in which he unmasked and denounced the Nestorian heresy.
For two years after the Council, Saint Celestine proclaimed the true teaching about Christ the God-Man, and he died in peace on April 6, 432.