HILARIUS, ST. (c. 3O0-367), bishop of Poitiers, was born of pagan parents at Poitiers, but later became a Christian, and about 353 was unanimously elected bishop of his native town. Almost at once, he secured the excommunication of Saturninus, the Arian bishop of Arles and of Ursacius and Valens, two of his prominent supporters, and wrote to the emperor Constantius a remonstrance against the persecutions by the Arians. But at the synod of Biterrae (Beziers), summoned in 356 by Constantius, Hilary was banished to Phrygia, continuing, however, to govern his diocese.
He wrote there the important De synodis or De fide Orien taliurn, an epistle addressed in 358 to the Semi-Arian bishops in Gaul, Germany and Britain, expounding the views of the Oriental bishops on the Nicene controversy, and the De trinitate. In 359 Hilary attended the convocation of bishops at Seleucia in Isauria, where, with the Egyptian Athanasians, he joined the Homoiousian majority against the Arianizing party headed by Acacius of Caesarea; thence he went to Constantinople, and through a peti tion (Ad Constantium Augustum liber secundus) personally pre sented to the emperor, was sent back to his diocese. In 364 he impeached Auxentius, bishop of Milan, and a man high in the imperial favour, as heterodox ; he was summoned to appear before the emperor Valentinian at Milan, but being unable to maintain his charges, was expelled from the city. In connection with the controversy, he published the Contra Arianos vel Auxentium Mediolanensem liber, and the Contra Constantium Augustum liber.
Hilary is sometimes regarded as the first Latin Christian hymn writer, but none of the compositions assigned to him is indis putable. Augustine termed him "the illustrious doctor of the churches:' His feast is celebrated in the West on Jan. 13. works were edited by Erasmus (Basel, 1523, 1526, 1528) ; P. Coustant (Paris, 1693) ; Migne (Patrol Lat. ix., x., • The Tractatus de mysteries, ed. J. F. Gamurrini (Rome, 1887), and the Tractatus super Psalmos, ed. A. Zingerle in the Vienna Corpus scrip. eccl. Lat. xxii. Translation by E. W. Watson in Nicene and Post-Nicene Fathers, ix.