Saint Hilary was a native of Poitiers in Aquitaine. Born and educated a pagan, it was not until near middle age that he embraced Christianity, moved to that step primarily by the idea of God presented to him in the Holy Scriptures. He soon converted his wife and daughter, and separated himself rigidly from all non-Catholic company, fearing the influence of error, rampant in a number of false philosophies and heresies, for himself and his family.
He entered Holy Orders with the consent of his very virtuous wife, and separated from his family as was required of the clergy. He later wrote a very famous letter to his dearly-loved daughter, encouraging her to adopt a consecrated life. She followed this counsel and died, still young, a holy death.
In 353 Saint Hilary was chosen bishop of his native city. Arianism, under the protection of the Emperor Constantius, was then at the heights of its exaltation, and Saint Hilary found himself called upon to support the orthodox cause in several Gallic councils, in which Arian bishops formed an overwhelming majority. He was in consequence accused to the emperor, who banished him to Phrygia. He spent his more than three years of exile in composing his great works on the Trinity.
In 359 he attended the Council of Seleucia, in which Arians, semi-Arians, and Catholics contended for the mastery. He never ceased his combat against the errors of the enemies of the Divinity of Christ. With the deputies of the council he went to Constantinople, and there so dismayed the heads of the Arian party that they prevailed upon the emperor to let him return to Gaul. He traversed Gaul, Italy and Illyria, preaching wherever he went, disconcerting the heretics and procuring the triumph of orthodoxy. He wrote a famous treatise on the Synods. After some eight years of missionary travel he returned to Poitiers, where he died in peace in 368.
Source: sanctoral.com