He was born at Poitiers about the end of the 3rd century A.D. His parents were pagans of distinction. He received a good education, including what had even then become somewhat rare in the West, some knowledge of Greek. He studied, later on, the Old and New Testament writings, with the result that he abandoned his Neoplatonism for Christianity, and with his wife and his daughter received the sacrament of baptism.
So great was the respect in which he was held by the citizens of Poitiers that about 353, although still a married man, he was unanimously elected bishop. At that time Arianism was threatening to overrun the Western Church; to repel the disruption was the great task which Hilary undertook. One of his first steps was to secure the excommunication, by those of the Gallican hierarchy who still remained orthodox, of Saturtuinus, the Arian bishop of Arles and of Ursacius and Valens, two of his prominent supporters.
About the same time he wrote to the emperor Constantius II a remonstrance against the persecutions by which the Arians had sought to crush their opponents (Ad Constantium Augustum liber primus, of which the most probable date is 355). His efforts were not at first successful, for at the synod of Biterrae (Beziers), summoned in 356 by Constantius with the professed purpose of settling the longstanding disputes, Hilary was by an imperial rescript banished with Rhodanus of Toulouse to Phrygia, in which exile he spent nearly four years.
Thence, however, he continued to govern his diocese; while he found leisure for the preparation of two of the most important of his contributions to dogmatic and polemical theology, the De synodis or De fide Orientalium, an epistle addressed in 358 to the Semi-Arian bishops in Gaul, Germany and Britain, expounding the true views (sometimes veiled in ambiguous words) of the Eastern bishops on the Nicene controversy, and the De trinitate libri XII, composed in 359 and 360, in which, for the first time, a successful attempt was made to express in Latin the theological subtleties elaborated in the original Greek. The former of these works was not entirely approved by some members of his own party, who thought he bad shown too great forbearance towards the Arians; to their criticisms he replied in the Apologetica ad reprehensores hurl de synodis responsa.
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, in a petition (Ad Constantium Augustum hiber secundus) personally presented to the emperor in 360, repudiated the calumnies of his enemies and sought to vindicate his trinitarian principles.
His urgent and repeated request for a public discussion with his opponents, especially with Ursacius and Valens, proved at last so inconvenient that he was sent back to his diocese, which he appears to have reached about 361, within a very short time of the accession of Julian.
He was occupied for two or three years in combating Arianism within his diocese; but in 364, extending his efforts once more beyond Gaul, he impeached Auxentius, bishop of Milan, and a man high in the imperial favour, as heterodox. Summoned to appear before the emperor Valentinian I at Milan and there maintain his charges, Hilary had the mortification of hearing the supposed heretic give satisfactory answers to all the questions proposed; nor did his (doubtless sincere) denunciation of the metropolitan as a hypocrite save himself from an ignominious expulsion from Milan.
In 365 he published the Contra Arianos vel Auxentium Mediolanensem liber, in connexion with the controversy; and also (but perhaps at a somewhat earlier date) the Contra Constantium Augustum liber, in which he pronounced that lately deceased emperor to have been Antichrist, a rebel against God, “a tyrant whose sole object had been to make a gift to the devil of that world for which Christ had suffered.”
Hilary is sometimes regarded as the first Latin Christian hymnwriter, but none of the compositions assigned to him is indisputable.
The later years of his life were spent in comparative quiet, devoted in part to the preparation of his expositions of the Psalms (Tractatus super Psalmos), for which he was largely indebted to Origen; of his Commentarius in Evangelium Matthaei, a work on allegorical lines of no exegetical value; and of his no longer extant translation of Origen’s commentary on Job.
He died in 367; no more exact date is trustworthy.
He holds the highest rank among the Latin writers of his century. Designated already by Augustine of Hippo as “the illustrious doctor of the churches,” he by his works exerted an increasing influence in later centuries; and by Pope Pius IX he was formally recognized as universae ecclesiae doctor (i.e. Doctor of the Church) at the synod of Bordeaux in 1851.
Hilary’s day in the Roman calendar is the 13th of January.
A "life" of Hilary was written by (Venantius) Fortunatus C. 550 but is not considered reliable. More trustworthy are the notices in Jerome (De vir. illus. 100), Sulpicius Severus (Chron. ii. 39-45) and in Hilary’s own writings.