For most of the Middle Ages, Christian thinkers were familiar with Plato and knew what aspects of Plato they accepted and which they didn't. It was only when the Crusades took place that Aristotle became known, and was considered suspect until St. Thomas Aquinas reconciled much of Aristotle with Christianity.
Luther, however was not convinced. He once remarked that "God had sent Aristotle as a punishment for the sins of mankind".