Answer
Hilary (315–367) was bishop of Poitiers in Gaul (located in modern western France). Hilary was a prominent orthodox voice during the Arian controversy.
Hilary was born into a pagan family in Poitiers but became a Christian around the age of 35. His spiritual growth must have been evident, for only three years later he was elected bishop of his hometown.
At the Council of Nicaea (AD 325), Arianism was roundly rejected, and the Nicaean Creed affirmed that the Father and Son were of the same substance—as the Father was divine, so was the Son. (According to the false teacher Arius, Jesus was “divine” but not in exactly the same way the Father was divine.) Athanasius was the leading and persuasive voice at Nicaea for the orthodox position against the Arian position.
However, in the years that followed Nicaea, Arius still had a following. In 337 an Arian emperor, Constantius II, came to power and sought to establish Arianism in the Roman Empire, by force if necessary. He called a church council in Milan (355) for the purpose of establishing Arianism. It was primarily attended by bishops who were sympathetic to Arianism, and as a result the council banished Athanasius.
Hilary of Poitiers was one bishop who boldly defended Athanasius and the orthodox position. After the Council of Milan, Hilary organized bishops in Gaul to oppose the Emperor and the Arian bishops. As a result, the Emperor exiled Hilary to Phyrgia in Asia Minor (modern-day Turkey). During that time Hilary wrote the most important systematic treatment of the Trinity at that time—On the Trinity. (The book is readily available online in English.) In his book, Hilary clarified key points that had been ambiguous after Nicaea. As a result many of the disparate Trinitarian and anti-Arian groups began to unify with this clearer understanding, and orthodox believers began to function more as a single body in regard to this critical doctrine.
Later, Hilary was allowed to return to Poitiers, but he was not restored to his position as bishop. He continued to oppose Arianism until his death in 367.
The following passage from On the Trinity, Book IV, gives a flavor of Hilary’s passion for the truth:
“It is with a full knowledge of the dangers and passions of the time that I have ventured to attack this wild and godless heresy, which asserts that the Son of God is a creature. Multitudes of Churches, in almost every province of the Roman Empire, have already caught the plague of this deadly doctrine; error, persistently inculcated and falsely claiming to be the truth, has become ingrained in minds which vainly imagine that they are loyal to the faith. I know how hardly the will is moved to a thorough recantation, when zeal for a mistaken cause is encouraged by the sense of numbers and confirmed by the sanction of general approval. A multitude under delusion can only be approached with difficulty and danger. When the crowd has gone astray, even though it knows that it is in the wrong, it is ashamed to return. It claims consideration for its numbers, and has the assurance to command that its folly shall be accounted wisdom. It assumes that its size is evidence of the correctness of its opinions; and thus a falsehood which has found general credence is boldly asserted to have established its truth.
“For my own part, it was not only the claim which my vocation has upon me, the duty of diligently preaching the Gospel which, as a bishop, I owe to the Church, that has led me on. My eagerness to write has increased with the increasing numbers endangered and enthralled by this heretical theory. There was a rich prospect of joy in the thought of multitudes who might be saved, if they could know the mysteries of the right faith in God, and abandon the blasphemous principles of human folly, desert the heretics and surrender themselves to God; if they would forsake the bait with which the fowler snares his prey, and soar aloft in freedom and safety, following Christ as Leader, prophets as instructors, apostles as guides, and accepting the perfect faith and sure salvation in the confession of Father and of Son. So would they, in obedience to the words of the Lord, ‘He that honors not the Son honors not the Father which has sent Him’ (John 5:23), be setting themselves to honor the Father, through honor paid to the Son.”
