Born into a wealthy pagan family in Poitiers, France, Hilary was well educated in the classics. As he looked into his own soul, however, he knew that he did not exist for the sole purpose of seeking pleasure, enjoying leisure, obtaining wealth, or merely satisfying his fleshly desires.
Hilary reasoned that the human soul did not exist simply to die. Instead, it must exist for something more, something eternal, something glorious. When his pagan culture did not suffice and philosophy fell short, Hilary finally found what he was searching for when he stumbled upon the Scriptures.
Hilary was first struck by the mysterious name of God in the Old Testament: “I AM WHO I AM.” God had revealed Himself as eternal, without beginning or end—Existence itself. Then Hilary discovered the Son of God in the Gospel of John 1:1–14. Of this discovery, Hilary said, “My soul measured the mighty workings of God, wrought on the scale of His eternal omnipotence . . .by a boundless faith . . .that God was in the beginning with God, and that the Word became flesh and dwelt among us . . .” (De Trinitate 1.12).
Hilary had the will to believe, and in the years that followed he was given the power and gift to understand the beauty, mystery, omnipotence, and nature of the Most Holy Trinity. Shortly after these discoveries of faith, Hilary was baptized a Christian and went on to defend the doctrine of the Trinity against the “insanity and ignorance of men.” He so impressed the faithful that they chose him to be their bishop, a dignity to which he reluctantly agreed.
Among those who shared in the “insanity and ignorance” of that time were a group of bishops and laity who followed the heresy of Arianism, which denied the divinity of Christ, instead holding that the Son was inferior to the Father. This heresy was especially strong in the Eastern Church but was starting to spread throughout France.
After Hilary was a bishop for only about five years, the emperor, an Arian himself, ordered every bishop to pledge their support for this heresy. Hilary refused. Instead, he vigorously defended the truth, and for his brave stance was exiled to Phrygia, in modern-day Turkey. In His love and providence, God used Hilary’s time of exile in powerful ways.
While in Phrygia, Bishop Hilary spent much time studying and writing. He had already composed a marvelous commentary on the Gospel of Matthew while in Poitiers, and now he set his mind to his greatest work, De Trinitate (On the Trinity). Drawing from his classical education, his knowledge of Greek, his love of the Scriptures, and from the “insanity” and “ignorance” of Arianism itself, Bishop Hilary composed a comprehensive defense of the doctrine of the Trinity as it was taught in the Nicene Creed. Bishop Hilary caused so much trouble for the Arians in Phrygia that the Arian bishops pleaded with the emperor to send him back home, a request the emperor honored.
On his return to Poitiers, Bishop Hilary took the long way home through Greece and Italy, preaching all the way, weeding out the beginnings of Arianism in the Western Church. His effectiveness came not only from his clear teaching, but also from his conciliatory approach and resolute determination. Back in Poitiers, he continued to preach, write, attend councils, and even to compose hymns. The hymns were his way of introducing the doctrines of the faith to the people of God in song. He was a true pastor who burned with a desire that everyone come to a deeper knowledge of the One God—Father, Son, and Holy Spirit.
Source: https://mycatholic.life/saints/saints-of-the-liturgical-year/january-13-saint-hilary-of-poitiers-bishop-and-doctor/
