What prompted the catholic reformation during the sixteenth century?

This question requires a lengthy answer to fully understand it, but I'll try to be as brief as I can so you don't fall asleep while trying to get through it.

The Catholic Church had grown very rich and very powerful over the centuries, and as such had also grown somewhat corrupt.  The Church believed only it had the power to interpret the Bible, a sentiment which many found to be insulting and outright false.  During the 14th century John Wycliffe declared that all people had the right to read and interpret the Bible for themselves.  Long story short: Wycliffe's writings and Biblical translations influenced a monk by the name of Martin Luther.

Luther had been protesting the the acts of some of the clergy who were basically selling "indulgences" without making it clear that those who partook must also be sincerely repentant for those sins.  Because of this Luther wrote a list of 95 theses outlining why this was a bad thing to do and nailed it to the door of the Wittenberg Church.

Eventually Luther became the leader of the Reformation and rejected the authority of the pope and set up the Bible as the sole source of Christian truth, a tenant followed by protestants today.  What this means is that neither priests nor other holy officials held any special power of the divine which the average person did.  Luther also rejected the idea of celibacy within the clergy and that the vows taken by monks and nuns were non binding.  

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