How did the power of the catholic church expand in the middle ages?

1. Providing a form of social cohesion prior to the rise of Nationalism. Islam and Christianity both fulfilled the same purposes in this regard. This allowed the Church to set the guidelines for morality and heavily influenced legality, making them as powerful and sometimes more powerful than the state. 

2. Giving the serfs a sense of hope in the hereafter, to alleviate their grievances in this life. By making them accept their lot in life it promised them paradise in the next. This was an essential factor in keeping them under control, something that the nobility needed to maintain. 

3. Being able to excommunicate was a dreaded punishment-not only did it ban you from an afterlife-anyone who supported you could also be punished. To your subjects-you were less important than the will of God, so if you were excommunicated, they would panic and might well revolt against you, as well as making you a social pariah.