This feast kept in Tours on January 13 from the end of the Fifth Century was inserted in the Roman Calendar by Pius IX.  St. Austin, who often urges the authority of St. Hilary against the Pelagians, styles him the illustrious doctor of the churches. St. Jerome says that he was a most eloquent man, and the trumpet of the Latins against the Arians; and that in St. Cyprian and St. Hilary, God had transplanted two fair cedars out of the world into his church.

St. Hilary was a native of Poitiers, and his family was one of the most illustrious in Gaul.  .  He spent his youth in the study of eloquence. He himself testifies that he was brought up in idolatry, and gives us a particular account of the steps by which God conducted him to the knowledge of his saving faith.  He considered by the glimmering or faint light of reason that man, who is created a moral and free agent, is placed in this world for the exercise of patience, temperance, and other virtues, which he saw must receive from God a recompense after this life. He ardently set about learning what God is; and after some researches into the nature of the Supreme Being, quickly discovered the absurdity of polytheism, or a plurality of gods, and was convinced that there can be only one God, and that the same is eternal, unchangeable, all-powerful, the first cause and author of all things. Full of these reflections, he met with the holy scriptures and was wonderfully affected with that just and sublime description Moses gives of God in those words, so expressive of his self-existence, I AM WHO AM; he was no less struck with the idea of his immensity and supreme dominion, illustrated by the most lively images in the inspired language of the prophets. The reading of the New Testament put an end to and completed his inquiries; and he learned from the first chapter of St. John that the Divine Word, God the Son, is coeternal and consubstantial with the Father. Here he checked his natural curiosity, avoided subtleties, and submitted his understanding to divine revelation, resolving what seemed incomprehensible into the veracity and power of God and not presuming to measure divine mysteries by his shallow capacity. Being thus brought to the knowledge of faith, he received the heavenly regeneration by baptism. From that time forth he so squared his whole life by the rules of piety, and so zealous were his endeavors to confirm others in the faith of the holy Trinity and to encourage all to virtue, that he seemed, though a laymen, already to possess the grace of the priesthood.
In the year 353 he was chosen bishop of Poitiers.  He omitted no endeavors to escape this promotion, but his humility only made the people the more earnest to see him vested with that dignity; and indeed their expectations were not frustrated in him, for his eminent virtue and capacity shone forth with such a lustre as soon drew upon him the attention, not only of all Gaul but of the whole church. Soon after he was raised to the episcopal dignity, he composed, before his exile, elegant comments on the gospel of Saint Matthew, which are still extant. Those on the Psalms he compiled after his banishment.  Of these comments on the Psalms and on St. Matthew, we are chiefly to understand St. Jerome, when he recommends, in a particular manner, the reading of the works of St. Hilary to virgins and devout persons.  From that time the Arian controversy chiefly employed his pen. He was an excellent orator and poet. His style is lofty and noble, beautified with rhetorical ornaments and figures, but somewhat studied; and the length of his periods renders him sometimes obscure to the unlearned, as St. Jerome takes notice.
The emperor Constantius, having labored for several years to compel the eastern churches to embrace Arianism, came into the West, and after the overthrow of the tyrant Magnentius, made some stay at Arles, while his Arian bishops held a council there, in which they engaged Saturninus, the impious bishop of that city, in their party, in 353. A bolder Arian council at Milan in 355, held during the residence of the emperor in that city, required all to sign the condemnation of St. Athanasius. Such as refused to comply were banished — among whom were St. Eusebius of Vercelli, Lucifer of Cagliari, and St. Dionysius of Milan, into whose see Auxentius, the Arian, was intruded. St. Hilary wrote on that occasion his first book to Constantius, in which he mildly entreated him to restore peace to the church. He separated himself from the three Arian bishops in the West — Ursacius, Valens, and Saturninus — and exhibited an accusation against the last in a synod at Beziers. But the emperor, who had information of the matter from Saturninus, sent an order to Julian, then Caesar, and surnamed afterwards the Apostate, who at that time commanded in Gaul, for St. Hilary’s immediate banishment into Phrygia, together with St. Rhodanius, bishop of Toulouse. The bishops in Gaul, being almost all orthodox, remained in communion with St. Hilary and would not suffer the intrusion of anyone into his see, which in his absence he continued to govern by his priests. The saint went into banishment about the middle of the year 356, with as great alacrity as another would take a journey of pleasure, and never entertained the least disquieting thought of hardships, dangers, or enemies, having a soul above both the smiles and frowns of the world and fixed only on God. He remained in exile somewhat upwards of three years, which time he employed in composing several learned works. The principal and most esteemed of these is that On the Trinity, against the Arians, in twelve books. In them he proves the consubstantiality of the Father, Son, and Holy Ghost. He teaches that the church is one, out of which all heresies spring, and that by this she is distinguished as standing always one, always alone against them all and confounding them all — whereas they by perpetual divisions tear each other in pieces, and so become the subject of her triumph.  He proves that Arianism cannot be the faith of Christ, not being the faith revealed to St. Peter, upon whom the church was built and secured forever; for Peter’s faith Christ prayed, that it might never fail; and it was Peter who received the keys of the kingdom of heaven and whose judiciary sentence on earth is that of heaven:  all which arguments St. Hilary frequently urges.  He proves the divinity of Christ by the miracles wrought at the sepulchres of the apostles and martyrs and by their relics; for the devils themselves confess Christ’s godhead and roar and flee at the presence of the venerable bones of his servants, which truth St. Hilary also mentions and urges in his invective against Constantius.  In 358, he wrote his book On Synods, or On the Faith of the Orientals to explain the terms and variation of the eastern Arians in their synods.
St. Hilary returned through Illyricum and Italy to confirm the weak. He was received at Poictiers with the greatest demonstrations of joy and triumph, where his old disciple, St. Martin, rejoined him, to pursue the exercises of piety under his direction. A synod in Gaul, convoked at the instance of St. Hilary, condemned that of Rimini, which, in 359, had omitted the word “consubstantial.” Saturninus, proving obstinate, was excommunicated and deposed for his heresy and other crimes. Scandals were removed, discipline, peace, and purity of faith were restored, and piety flourished.
Our saint died at Poictiers in the year 368, on the thirteenth of January or on the first of November, for his name occurs in very ancient Martyrologies on both these days. In the Roman breviary his office is celebrated on the fourteenth of January.
St. Hilary observes, that singleness of heart is the most necessary condition of faith and true virtue, “For Christ teaches that only those who become again as it were little children, and by the simplicity of that age cut off the inordinate affections of vice, can enter the kingdom of heaven. These follow and obey their father, love their mother; are strangers to covetousness, ill-will, hatred, arrogance, and lying, and are inclined easily to believe what they hear. This disposition of affections opens the way to heaven. We must therefore return to the simplicity of little children, in which we shall bear some resemblance to our Lord’s humility.”  This, in the language of the Holy Ghost, is called the foolishness of the cross of Christ, in which consists true wisdom. That prudence of the flesh and worldly wisdom which is the mother of self-sufficiency, pride, avarice, and vicious curiosity, the source of infidelity, and the declared enemy of the spirit of Christ, is banished by this holy simplicity; and in its stead are obtained true wisdom — which can only be found in a heart freed from the clouds of the passions — perfect prudence — which, as St. Thomas shows, is the fruit of the assemblage of all virtues — and a divine light which grace fails not to infuse. This simplicity, which is the mother of Christian discretion, is a stranger to all artifice, design, and dissimulation, to all views or desires of self-interest, and to all undue respect or consideration of creatures. All its desires and views are reduced to this alone: of attaining to the perfect union with God. Unfeignedly to desire this one thing — to belong to God alone, to arrive at His pure love, and to do His will in all things — is that simplicity or singleness of heart of which we speak, and which banishes all inordinate affections of the heart, from which arise the most dangerous errors of the understanding. This is the essential disposition of everyone who sincerely desires to live by the spirit of Christ. That divine Spouse of souls loves to communicate Himself to such.  His conversation (or as another version has it, His secret) is with the simple.  His delight is in those who walk with simplicity.  This is the characteristic of all the saints:  for which reason the Holy Ghost cries out, “Approach Him not with a double heart.”  That worldly wisdom is not subject to the law of God, neither can it be.  Its intoxication blinds men and shuts their eyes to the light of divine revelation. They arrogate to themselves the exclusive privilege of learning and clear understanding, but the skepticism, the pitiful inconsistencies, and monstrous extravagances, which characterize their writings and discourses, make us blush to see so strong an alliance of ignorance and presumption and makes us lament that the human mind should be capable of falling into a state of so deplorable a degeneracy. Among the fathers of the church we admire men the most learned of their age, the most penetrating and most judicious and, at the same time, the most holy and sincere, who, being endowed with true simplicity of heart, discovered in the mysteries of the cross the secrets of infinite wisdom which they made their study, and the rule of their actions.
(Adapted from Butler’s Lives of the Saints and Fr. Lasance, The New Roman Missal)