Caligula has a point, and there is a symmetry to it as well. When you have a single strong church, people who want to dissent have to do it from within.
When Europe was close to 100% Catholic, except for some Jews who were damned good at hiding, the early protestants started actually reading the Bible themselves. Printing presses and the translation into modern languages sped this.
By this point the Catholic church had "interpreted" the Bible so thoroughly that it had drifted quite a bit. To an extent it was necessary, since for a start there is slight Biblical justifcation for it's very existence.
Protestants tried to go back to the original texts, which in the case of Christianity is a bit hard because there aren't any. But they tried quite hard to be fundamentalists, reading the Bible directly, which in some cases (like Henry VIII's England) meant that gits like St. Thomas More would have you tortured then burned.