The professor's letter was polite and professional, with a formal tone that one might find in a modern academic work. It could hardly be described as revolutionary.

Yet within a few years, the professor would find himself excommunicated by the Pope, branded an outlaw and heretic and living in hiding under the protection of an army of followers.

His name, of course, was Martin Luther. And the publication of his famous paper, the 95 Theses, is often viewed as the start of the Protestant Reformation, one of the most important social movements in history.

The reformation was a European-wide rejection of Church authority. But its origins far predate Luther or his 95 Theses.

Mini revolts against the Church go back to the late 1300s, more than 150 years earlier.

But historians often focus on a single watershed moment to mark the beginning of a major movement or trend, even though there are always multiple events leading up to it.