What led to a decrease in power of the catholic church during the 15th century?

Reformation: The Protestant Reformation caused people to leave the Catholic Church reducing its power.

In 1517, Martin Luther began a revolution within the Catholic Church creating a new religion and spurring others to break away as well. The branches of the Protestant Church rose out of the movement reducing the membership of the Catholic Church. This reduced the money and therefore the power of the Church. 

Political forces often influence public opinion.