Skepticism.
The Enlightenment was a social movement during the 18th century, that came right after the Scientific Revolution of the 17th century. The reason why it is important to mention the correlation between these two is because the Enlightenment was a direct product of the Scientific Revolution which was a period of abundant developments in sciences such as Mathematics, Biology, Physics, and Astronomy.
This abundance of discoveries about nature completely changed the way society thought about all phenomena. It defied the previous belief system that stated that everything could only be explained through religion. This radical change in perception created a sense of skepticism towards religion that started the momentum for the Enlightenment movement to develop and to make a place for concepts such as reason, and liberty. All these new ideals armed with the, then, newly developed Scientific Method, paved the way for important movements of independence from Aristocracy establishments which were essentially founded on religious beliefs.