One of the most important Enlightenment concepts was the notion that the truth about the world around us should be based on empirical evidence. Enlightenment thinkers believed that the kind of empirical investigation used in scientific inquiry was the surest and most secure method of finding out about the natural world. This meant that science, if allowed to develop freely, would often reach conclusions that conflicted with the teachings of the Church. This is precisely what happened with Galileo, for instance.
Although not all churchmen were hostile to Enlightenment principles, a fair number of them saw these principles as a threat to the Church's monopoly of the truth. The Catholic Church regarded itself as the sole repository of truth, both spiritual and temporal: a truth whose interpretation was determined by the Church itself. For Enlightenment thinkers, however, truth was its own authority; it did not have to rely on the Church or any other institution for its support. Enlightenment principles, then, posed a serious threat to the power of the Catholic Church and, indeed, all forms of organized religion that believe themselves in possession of the absolute truth.