The beginning of the modern disbelief.
The Decline of Christendom With the rise of Science and philosophy.
Apart from the fragmenting of Christendom, science, philosophy and secularism and materialism played their parts in raising doubts and fostering skepticism about God and religion.
New scientific knowledge questioned many of the Church’s teachings like the geocentric doctrine, ( Copernicus established beyond doubts that the earth and the other planets was rounding the sun) many of the natural phenomena like the thunder and lightening, and the appearance of certain comets were scientifically explained. Many “miracles” were explained and all of sudden God and religion seemed outdated and retrograde.
The theory of evolution (Darwin published the his Origin of Species in 1859) was the severest blow to the Catholic Church. At first the clergy denounced the theory. But before the scientific evidence opposition soon faded and evolution became compatible. In so doing it undermined the faith in God.