Have you ever been told that any issue of “faith is a private matter and should be kept to oneself.” Where did this idea come from?
Both Bacon and Descartes trusted in reason to be the arbiter of truth (Read “Fact versus Truth“) albeit from different starting points. Bacon used reason to take him from observation of particular phenomenon to universal principles, and Descartes saw the human mind as the final authority in understanding reality. Although they approached it from different angles, both trusted reason, rather than faith and tradition, to lead to the truth.
Because of their influence, by the middle of the 17th century, science was becoming the lens by which reality was viewed. Importantly, this does not mean that there was a corresponding loss of belief. Still, as the mysteries of nature that had previously been attributed to the direct intervention of God came to be explained as natural phenomenon, a division developed between science and religion. God was understood to be the creator, but was no longer thought to be necessary for day to day management of the material world because it was obedient to Natural Law. Correlative to the division between God and His Creation, was a widening gap between God and human reason; reason was understood to be autonomous.
Enter Immanuel Kant (1724-1804). Kant saw the movement from reliance on God toward a reliance on reason as analogous to the movement from childhood to adulthood. This idea was foundational to the period we call the Enlightenment. The light of the Enlightenment was the realization that it was neither God nor the church which would lead to a better world, but human Reason. This view of is the essence of the modern worldview, and is still with us today.
Kant believed that human beings were also developing morally as we continue to articulate universally recognized moral principles. All cultures and religions are expressions, to one degree or another, of these principles. He believed that these Moral Laws could be uncovered by reason. For Kant, religion was simply a particular expression of universal principles.
The light, in Enlightenment, is Reason. It was supposed that we could arrive at universal truth using only reason. Importantly, it was believed that reason was neutral, unaffected by belief, (or history, tradition, body, etc.). It wasn’t very long before religion was thought to be its opposite.
This is where the divide between faith and reason was formalized–this is dualism. It’s the belief that we can hold to whatever particular beliefs we want, but these are to be kept in the private sphere. The public sphere is to be ruled by universal reason. If we keep things in their proper spheres, we can all happily get along (This false dichotomy, and others, is the point of this site).
Although, this idea is considered passé by many intellectuals–not just the religious ones either–it still dominates public thought.