In a nutshell, the Reformation happened because there has been a constant pull against the Catholic Church since Pentecost, but it wasn’t until the fall of Rome, the Black Death, and the rise of Islam that Catholic Europe became weak enough to succumb to an attack from within.
When the Thirty Years War resulted in a bloody stalemate and an exhausted Europe, the Reformers essentially won.
Belloc argues (and I agree) that the reason the Reformation happened when it did was simple greed—the Catholic Church had accrued wealthy lands by this point, and the minor nobility wanted them.
This was most prevalent in England, where a whole class of millionaires made by stealing monastic lands fueled Elizabeth’s rise to the throne and paved the way for Protestant England.
The doctrinal disputes came later, once Calvin published his book about 20 years into the Reformation (Luther nails up his theses in 1517; Calvin publishes in 1536). It is often taught (when taught at all) in reverse order.
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