I think it depends somewhat on a subquestion I'm confused about. How much culpability should we assign the Church as an institution for the Reformation? On the one hand they were getting pretty corrupt, on the other hand that's like blaming someone who lived a vigorous, moral life, but who is now dying of cancer, for harboring cancer. Should we blame the man for not having already discovered the cure to cancer? Anyway, my intuition says the answer is about 1200 or 1300 A.D., but I really don't know. How close or far before the Reformation is dependent on how culpability should be assigned to the Church for the Reformation. JaysonVirissimo or VladimirM would have better answers.

Sorry; my knowledge of the Middle Ages (and the Early Modern Period) is very low-level (with depth on very narrow topics like medieval science and logic, but little outside of that including politics and religion). Making an accurate judgment as to the (average?) truth-value of the many (importance-weighted?) propositions affirmed by (the majority of?) Catholic churchmen is way too high-level for my current understanding (although, I hope to rectify this in the near future). Also, although many of my comments can reasonably be interpreted as being "pro-Catholic", this is mostly by accident. It would be more accurate to say that I am defending the medievals (many of which were Catholics) from libel (of which I have been guilty in the past and am attempting to do penance).