The crusades, which marked a dark period in the history of Christianity, dominated the church's external relations from the end of the 11th century until the middle of the 15th century. With their brutality, misdirection and violent conquests, and the lust for power and expansion that they engendered, the Crusades damaged the church's relations with the Jewish and Muslim worlds, if not irreparably then at least until the late 20th century. Their legacy also served to seal the rift between the Orthodox Church of the East and the Latin Church of the West.

Yet, despite this stain on the heritage of Christianity, internally the Latin Church of the West was lifting itself out of the Dark Ages. Once again, the joys of philosophy were being rediscovered, and expressed in the writings of great Christian thinkers such as Anselm, Bernard, Peter of Lombard, and, uniquely, Thomas Aquinas.

At the same time, the monastic orders were being challenged by the reforming zeal of the Cistercians, while new orders such as the Franciscans and Dominicans were challenging many of the weaknesses corrupting the church. And fresh stirrings and new ideas from Peter Abelard, the Waldensians and John Wycliffe were a hint at, or foretastes of, the great movement that would soon challenge the institutionalised Church through the Reformation.

At the time the first Crusade was being proclaimed, the Archbishop of Canterbury was the former Benedictine Abbot of Bec, Anselm (ca 1033-1109), who spent most of his reign in exile on the Continent. Anselm was the first truly great theologian of the mediaeval West, and is sometimes described as the founder of scholasticism. He followed Augustine's method of "faith seeking understanding" and allowed philosophy to play a significant, if limited, role in theology.

In his Proslogion Anselm presented his famous "ontological argument" for the existence of God. Today, it would be easy to accuse him of attempting to define God into existence, but despite their weaknesses Anselm's arguments are impressive for their day. He sought to show how reasonable faith is, rather than to offer a strict proof of it and he succeeded in bringing theology back to the level of debate it had lost since the days of Gregory the Great 500 years earlier.

Bernard of Clairvaux (10901153), who preached round Europe raising support for the second Crusade, had first entered the new Cistercian monastery of Clairvaux to flee the world, but became one of the most widely-travelled and active leaders of the Western Church in the 12th century. The last great representative of the early medieval tradition of monastic theology, Bernard has been called "the Last of the Fathers". He was a strong opponent of Peter Abelard and the Waldensians, and with equal vigour defended the claims of the Papacy.

But he also warned against the dangers of papal tyranny. "It seems to me you have been entrusted with stewardship over the world, not given possession of it", he told Pope Eugenius III. "There is no poison more dangerous for you, no sword more deadly than the passion to rule."

Bernard's contemporary, Peter Lombard (ca 1100-1160), who died as Bishop of Paris, was the author of the Sentences, which became the standard textbook of theology. Writing a commentary on his Sentences became a regular part of the preparation for a doctorate in theology, and it was generations before his work was superseded by the Summa Theologica of Thomas Aquinas.

Thomas Aquinas (1225-1274) spent most of his life teaching at Paris, where the burning issue was how theology should respond to the rediscovery of Aristotle. His contemporary, Bonaventure (1221-1274), kept to the traditional Platonist worldview, while Thomas tried to conduct a synthesis between reason and faith, philosophy and theology, Aristotle and Christianity.

His Summa Theolgica was written in the last 10 years of his life, and it took some centuries for it to replace Peter Lombard's Sentences as the standard textbook for Western theology. Despite a decline in Thomas's influence in recent decades, his work remains the greatest achievement of scholastic theology, and his method of reasoning and approach to philosophy has influenced subsequent generations of philosophers, including Marx.

However, his spiritual greatness should not be forgotten either: near the end of his life, he had a vision while saying Mass that caused him to stop writing; he stated that compared with what had then been revealed to him, all that he had written seemed like straw.

Many of his contemporaries were equally humble, and denied they had contributed any original thoughts to the fields of theology or philosophy. These great men, in their humility, believed they were simply building on the works and writings of their predecessors, or, in the words of Bernard of Chartres: "We are like dwarves sitting on the shoulders of giants".

But apart from their writings, which influenced theology and philosophy for centuries after, Bonaventure, Thomas and Bernard also represented three new forms of monastic life which continue to shape the spirituality of the church: Bernard was instrumental in the spread of the Cistercians, who sought to reform the Benedictine tradition; Thomas was a member of the Order of Preachers, the Dominicans founded in 1216 by Dominic (1170-1221); while Bonaventure was Minister General of the Franciscans, founded by Francis of Assisi (1181-1266).

However, not all the great spiritual writers of the day were men. In recent years there has been a renewed interest in the writings of the English mystic and anchoress, Julian of Norwich (c 1342 after 1413).

Despite the cruelty of the Crusades, and the relentless pursuit of dissent in the shape of the Albigensians and the Waldensians, the spirituality of Julian and of Thomas a Kempis, the theology of Aquinas and the poverty of Dominic and Francis point to a Christianity that continued to develop new riches and thinking.

Although the integrity of the western church was weakened by the Crusades and its claims further weakened by the Avignon captivity of the Papacy (1309-1377), Western Christianity was alive intellectually and spiritually.

The questioning faith of Peter Abelard in France in the 12th century, the Waldensians in Italy and further afield in the 13th century, and of John Wycliffe and the Lollards in England in the 14th century were nurtured in a church that would soon find itself ripe for the challenges posed by both the Reformation and the Counter-Reformation.

Rev Patrick Comerford is a writer on church history and theology and an Irish Times journalist. Contact: theology@irish-times.ie loughlin/index.html