The fourth chronologically among the Doctors of the Church, was Saint Hilary of Poitiers, the first Aquitaine to be honored by the Church. He was officially recognized as a Doctor by Pope Pius IX in 1851.
He was born in Poitiers in Gaul in what is today France the same year as Saint Cyril of Jerusalem - 315. Like Cyril, he became a bishop and, like Cyril, he was persecuted during the great Arian heresy in the fourth century.
Unlike Cyril, Hilary came to Christianity later in life. Raised as a pagan, he found a challenge in Sacred Scriptures and there found the One True God. Married during his early years, he soon converted his wife and daughter who enthusiastically embraced the faith. Because celibacy for clerics was not yet mandatory Hilary was ordained in 347 and, three years later, at the age of 35, the people of Poitiers elected him as the new bishop of their city. As so many saints are wont to do because of their humility and heroic charity, Hilary objected but they insisted. Obediently he agreed. Immediately he was thrown into the thrust of the Arian controversy.
In 355 he was summoned to the a synod in Milan by the Roman Emperor Constantius. Hilary knew that most of the bishops there would be Arians and that their main purpose was to condemn the holy Saint Athanasius and therefore he refused to be part of the blackballing. Because of his resistance, he himself was condemned by his fellow bishops in Gaul at a Synod in Beziers a year later. This council was presided over by the overt Arian Bishop Saturninus of Arles. Exiled a year later by Constantius, he was sent to Phrygia.
It was there that he studied more, becoming one of the great theologians of his time, and where he wrote his great works on the Trinity, most notably De Trinitate, De synodis and Opus historicum, all intended to defend orthodoxy against the cancerous Arianism.
After three years in exile, he attended the Council at Seleucia in 359 where, despite a power struggle among Arians, semi-Arians, and a loyal Catholic contingent, he was successful in convincing many of the clergy to embrace orthodoxy and resist the Arian bishops. The resistance was so strong that the latter convinced the Emperor to end his exile and send him back to Gaul just to be rid of his orthodoxy, so great his influence in Phrygia.
In 360 the Emperor ended his exile and returned him to his native Aquitaine. A year later he helped call a Synod together where Saturninus was soundly condemned and stripped of all power. With the death of Constantius in 361, and Saturninus out of the way, Hilary determined to root out more Arian bishops. He traveled throughout Gaul, Illyria and Italy where, in Milan in 364 he publicly debated the Arian bishop Auxentius. Despite his powerful words and the support of many, Auxentius had the protection of the new Emperor Valens who ordered Hilary out of Italy and back to Gaul.
Hilary took his time returning to his native land. His missionary spirit and zeal kept him on the road, so to speak, converting many more by his simple orthodoxy and mastery of the Faith. A few years after returning to his beloved city of Poitiers, he passed on to his Heavenly reward in 368.
This saint, who is considered the "Laborer for God" because of his work Opus historicum, labored willingly and joyfully, as well as fiercely in defending the Truths and Traditions of Holy Mother Church.
Note: [editor's bold, brackets and italicized for emphasis]