It was a period of darkness in the Church. Some historians describe it as a time in which the least went on in the Church and attacks upon it were greater than the Reformation. Ironically, this Age of Reason has its roots in Catholic thinkers who modern secularists have canonized; Descartes, Rousseau, Voltaire and other French Catholic thinkers. They were instrumental in moving Europe away from its Christian roots. They were preoccupied with knowledge of math and science, moving us from a God centered world to a man centered world.
The Jesuits, powerful defenders of the Faith were suppressed from 1773- 1814. They were outlawed in France and then throughout the world. This further weakened the Church defense against the rationalists.
The French Revolution 1789-96 was the culmination of the enlightenment to eliminate religion. Only six French Bishops of 134 took an oath to abolish the Church unlike the English Church under Henry VIII where only 2 Bishops refused to support Henry. 45% of French priests took the oath and those who did not were martyred. By 992 only 50% of the clergy were left and there were no ordinations.
Napoleon came on the scene in 1796, reinstated the Church but wanted to control it. The concordat of 1801 required the Church to take an oath of allegiance to the State. The Pope excommunicated Napoleon.
This period is considered to be the rise of modernism which attacked the meaning and value of Faith. It was atheistic to the core. Society lost control of itself, lost self-restraint, lost rhetoric and logic and became centered on the emotion not truth. It led to moral relativism. Violence in Rome lead to the Pope fleeing the city for two years and the government took over the Papal States. On his return the Pope became a prisoner of the Vatican.
Later in nineteenth century the 20 Church Council took place, Vatican I from 1869-70. It was the first ecumenical Council. Communism was condemned.