The Enlightenment, meaning the period that took up most of the 18th century is also known as the Age of Reason. As the influence of the Church was already on a decline in the 17th century and challenged by scientific discoveries, a movement was created pushing the importance of reason and science.
Challenging the authority and dogmas of the Church, the intellectuals of the 18th century lay the groundwork of our current society of liberty and tolerance.
What lead to the Age Of Reason
The scientific discoveries in the 16th and 17th century made by scientists including Copernicus, Galilei, Newton, led to a change in the understanding of our universe. People started to see the benefits of a more scientific method of understanding everything, instead of accepting the creation story of the Church and the dogmas. As people started to question everything and accept a more rational way of thinking, it reflected on the whole of society, including philosophy, science, politics, and culture.
Rational thinking
Inspired by leading philosophers like Voltaire, Rousseau, Locke, ... old medieval, theological methods were no longer applied, and people favored research through rational questioning. More and more philosophy strongly influenced politics in society, and separation of powers in the government was advocated by political thinker Montesquieu. His views provided the framework that was incorporated in the United States Constitution.
The fundamental values of equality of all men, the rights of the individual and a political system in which the people must be represented had their origins in the 18th century.
The scientific revolution that started with the theory of Copernicus of a sun-centered universe, made a lot of progress in the Age of Reason. Maybe one of the scientists with the most influence on modern day science is Isaac Newton, who described the law of gravitation. His thesis proved that we can describe the universe in mathematical rules, and not only religion could explain the unknown mysteries out there.
The general population was getting more and more literate, and in a search for knowledge, science was made popular with papers and publications. The most famous being the Encyclopédie, a general encyclopedia which was made by multiple authors and that had the goal to collect all knowledge of the world. The work was made of 28 volumes and contained around 70000 different articles about science, arts, and crafts.
Religion
As people got more educated, Christianity lost a big part of its influence. Deism, the belief in a God that did not interfere with day-to-day life, was getting more followers and even Atheism was heavily discussed, although there was the belief that atheists were a danger to the order of society and most intellectuals found it rational to believe in a higher power.
The end
The French Revolution in 1789 marks the end of the Enlightenment and was followed by a counter-reaction, Romanticism. Instead of relying on the rational thought, a return to the emotions, imagination, and spontaneity of the medieval way of thinking was longed for, and the past was glorified.