University of Naples, 6 December 1273. A Dominican friar and scholar is working on the final section of his life's culminating work. He is about 48 years old, and has written many biblical and philosophical commentaries and theological treatises. He goes to celebrate mass, and there he experiences a crisis – a mystical experience, or possibly a stroke. "All I have written seems as straw compared to what I have experienced," he says, and he abandons his masterpiece unfinished. A few months later, he falls ill on a journey to a church council in Lyons, and he dies at the monastery at Fossanova on 7 March 1274. The man is Thomas d'Aquino, known to us as Aquinas, and the work is his Summa Theologiae.

The 13th century was a turbulent era of crusades, religious conflicts and power struggles. Two of Aquinas's brothers were caught up in wars between Emperor Frederick II and the papacy, and one was executed for treason. Wars were fought in the name of religion, such as the fourth crusade and the Albigensian crusade. However, it was also one of the most exciting intellectual eras in European history.

Learning had shifted from monasteries and cathedral schools into the newly established universities. Muslim scholars had brought Arabic translations of Greek texts into the west, and their subsequent translation into Latin introduced Christian scholars to the works of Aristotle and others. Christian doctrine was encountering a wide range of competing beliefs, including those of Greek philosophy and its Jewish and Muslim interpreters. Aquinas brought to that era a synthesising brilliance with regard to texts and ideas which has left a deep imprint on western religion, politics, law and ethics.

Unlike his warring brothers, Aquinas was happier reflecting upon life than immersing himself within it. It was ideas, not politics and current affairs, that captured his imagination. As a young man he defied his family's wishes that he should enter the Benedictine order, opting instead to join the recently established Dominicans or Order of Preachers, which he had encountered during his studies in Naples in 1242-43. His family arranged for him to be kidnapped and imprisoned and they even sent a prostitute to seduce him (nobody explains why seduction by a prostitute might make him more inclined to join the Benedictines than the Dominicans). Aquinas resisted and eventually his family capitulated.

GK Chesterton described him as "a huge heavy bull of a man, fat and slow and quiet, very mild and magnanimous but not very sociable". His fellow friars referred to him as "the dumb ox", to which his teacher Albert the Great responded that "the dumb ox will bellow so loud that his bellowing will fill the world". Aquinas was a man of profound humility and prayerful contemplation, but he was also a pioneering genius whose writings constitute the apotheosis of medieval thought and the embryonic beginnings of modernity.

Aquinas's apparent indifference to the crusades and his pragmatism with regard to violence and suffering might shock modern sensibilities. However, our own era is also divided between violent conflicts and great intellectual achievements, and there are many scholars who show little interest in war and politics, and yet who produce works of scientific, philosophical and artistic genius. In detaching himself from the turmoil surrounding him, Aquinas was able to dedicate himself to his life's mission.

Through a close engagement with Aristotelian philosophy, he wanted to demonstrate that faith and reason, philosophy and theology, could be united in a mutually beneficial marriage within the overarching goodness of a cosmos created, ordered and sustained in all its aspects by God. Like all marriages of Aquinas's time, this was not a partnership of equals, for philosophy was theology's handmaid. Ultimately however, faith and reason, grace and nature, went together like love and marriage or horse and carriage, and together they could guide the human mind in its desire for God and for truthful knowledge about the world.

A number of obstacles must be overcome if we are to appreciate Aquinas today. In Protestant cultures he remains associated with an era that many believe to have been mired in barbarism and superstition, notwithstanding the magnificence of the medieval legacy, from the great cathedrals of Europe to the rise of the universities. The influence of modern scientific atheism has led to the widespread belief that one must choose between faith and reason, and that faith is fundamentally irrational and opposed to science. This is an idea that Aquinas dedicated his life to resisting. If we can set aside our prejudices in order to approach him afresh, we may be surprised at how relevant he still is.

• Follow Comment is free on Twitter @commentisfree