Christianity became the dominant religion of the Roman Empire early in the 4th century. For the next 1,000 years it dominated philosophy and tolerated little opposition. The chief philosophers were churchmen, especially teachers of theology.
Platonism and some elements of Neoplatonism were absorbed and used by Christian teachers and blended with Biblical doctrine. Early Christian philosophy begins with Augustine of Hippo and includes Boethius, the church fathers, Anselm of Canterbury, Bernard of Clairvaux, and Peter Abelard. With the rediscovery of Aristotle, largely through the writings of Muslim philosophers in the 12th century, his influence became dominant for a time in Western Europe and reached its pinnacle in the teachings of Thomas Aquinas.
Augustine identified the eternal ideas of Plato with truths that come from God. This divine world of truth is encountered by turning the mind toward God's revelation. Augustine taught that the immortality of the human soul can be proved by its possession of eternal truths.
Boethius was a major channel of Platonist philosophy to the Middle Ages. In 'The Consolation of Philosophy' he teaches that the eternal ideas are inborn ideas that people remember from the previous existence of the soul.
Between Augustine and Aquinas the pivotal character in philosophy was Anselm. He used both faith and reason to arrive at truth. He is most remembered for his proofs of the existence of God, derived from Neoplatonist philosophy. Bernard of Clairvaux was suspicious of building faith on philosophical concepts. He developed a doctrine of mystical love as the path to truth. Abelard constructed a question-and-answer method for teaching theology, published in his book 'Sic et Non' (Yes and No). His main interest was in logic. He taught that the material world is real. Universal ideas, in contrast to Plato, are only names or mental concepts. This position, called nominalism, had great influence in sidetracking Platonism from its dominant position in philosophy.
During the 12th century a revolution took place that completely changed the course of Western philosophy. The writings of Aristotle were translated into Latin and were studied by churchmen for the first time. They gave teachers access to his scientific works and to his logical method of argument. Many of these Latin translations are based on earlier Arabic translations and commentaries by such Muslim writers as Avicenna and Averroes.
The 'Metaphysics' of Aristotle was especially influential in turning philosophers away from Plato. The scientific writings prompted research into the natural world by such men as Roger Bacon.
Medieval theologians who sought to reconcile the doctrines of Christianity with the rational explanations of the world given by Aristotle were called Schoolmen, or Scholastics, because they were university teachers. Their philosophy is called Scholasticism. This merging of Aristotle with doctrine culminated in the writings of Thomas Aquinas, one of the great system builders in the history of philosophy. His major work is 'Summa Theologica' (Summary of Doctrine), a question-and-answer approach to teaching that has never been equaled. He posed questions, stated objections, then presented replies to every objection. Aquinas attempted to settle the conflict between faith and reason by showing that reason should deal with the facts of nature, but that supernatural truths of revelation must be accepted by faith. He said that some truths, such as the existence of God, are both revealed and provable by reason. Opposition to his teachings came from John Duns Scotus, William of Ockham, and others. Opposition to Aquinas was condemned by the Roman Catholic church, but it persisted.
By the 14th century there was a revival of Platonism and Neoplatonism in writers such as Meister Eckehart and Nicholas of Cusa. Aristotelianism lost its vitality, but its impact had been made. While theology persisted with Platonic ideas, the natural sciences and other research continued the path Aristotle had pioneered. Soon even it was overtaken by a period of invention and discovery that pushed medieval philosophy and other studies aside.