Economically, Europe was isolated. Most people lived their whole lives on the manor (piece of land owned by a lord) because they could grow all of the food they needed there, the manor was self-sufficient. There was little trade between Western Europe and the rest of the world and as a result, there was a lack of new ideas and innovation. 

Scientists in the 1600s and 1700s discovered natural laws, facts proven true through the scientific method, that disproved what the Catholic Church claimed. For example, Galileo Galilei, an astronomer whose ideas were based on an earlier scientist named Nicolaus Copernicus, wrote about and taught that the sun was at the center of the solar system, an idea called the heliocentric theory. The church claimed that the Earth was at the center and that the sun revolved around it. In 1633, Galileo was convicted of heresy, teaching an idea the was in opposition to the church, was forced to state that he was wrong, and spent the rest of his life under house arrest. 

Many other discoveries were made during the Scientific Revolution that changed human history including Isaac Newton’s discovery of the laws of motion and gravity and advances in every field of science and technology. 

In the Middle Ages, the Catholic Church was the cultural, intellectual, and political center of Europe. Monarchs were crowned by the Pope, most art and music was about Christianity and paid for by the Church, and the church was the source of all knowledge. 

The Bible, which was the source of the Catholic Church’s power and knowledge and it was only written in Latin, a language that very few people outside of the clergy (educated members of the church) could read, so priests interpreted the Bible for everyone else and told them what it said they should do and how they should act. 

The Protestant Reformation changed all of that. In 1517, a Catholic monk named Martin Luther started the Protestant Reformation by writing, publishing, and distributing his critiques of the church’s selling of indulgences called the 95 Theses. 

Martin Luther believed that the only true word of God was the Bible and that people did not need a pope, bishops, or priests to interpret it for them. He felt that the Catholic Church had corrupted what the Bible actually said and that the selling of indulgences was just one example. Indulgences were pieces of paper issued by the Catholic Church that forgave sins.
In response, Luther had the Bible printed in the languages that most people spoke and created a new church called the Lutheran Church. Later, many other Protestant churches were created by other leaders of the Reformation. 
Many monarchs, especially in Northern Europe, converted to Protestant religions as a way of freeing themselves from the power the Catholic Church held over them since the Middle Ages.

Source: Adapted from  http://www.biography.com/people/martin-luther-9389283#later-year