Bishop, born in that city at the beginning of the fourth century; died there 1 November, according to the most accredited opinion, or according to the Roman Breviary, on 13 January, 368. Belonging to a noble and very probably pagan family, he was instructed in all the branches of profane learning, but, having also taken up the study of Holy Scripture and finding there the truth which he sought so ardently, he renounced idolatry and was baptized. Thenceforth his wide learning and his zeal for the Faith attracted such attention that he was chosen about 350 to govern the body of the faithful which the city had possessed since the third century. We know nothing of the bishops who governed this society in the beginning. Hilary is the first concerning whom we have authentic information, and this is due to the important part he played in opposing heresy. The Church was then greatly disturbed by internal discords, the authority of the popes not being so powerful in practice as either to prevent or to stop them. Arianism had made frightful ravages in various regions and threatened to invade Gaul, where it already had numerous partisans more or less secretly affiliated with it. Saturninus, Bishop of Arles, the most active of the latter, being exposed by Hilary, convened and presided over a council at Béziers in 356 with the intention of justifying himself, or rather of establishing his false doctrine. Here the Bishop of Poitiers courageously presented himself to defend orthodoxy, but the council, composed for the most part of Arians, refused to hear him, and being shortly afterwards denounced to the Emperor Constantius, the protector of Arianism, he was at his command transported to the distant coasts of Phrygia.
(Taken from Catholic Encyclopedia)