Something that Artog said reminded me of the Reformation. Yeah. I was a medieval historian. These things happen from time to time.
Martin Luther. Architect of the doctrine of justification by faith alone, which inadvertently caused a schism from the Catholic Church and resulted in centuries of bloodshed. He didn't come to these radical conclusions overnight, you know: he suffered terrible spiritual agonies over where his thinking was taking him. He didn't take this stuff lightly AT ALL, but his conscience told him that the Bible was the only source of divinely revealed knowledge, that everyone should be able to read it in their own mother-tongue and that you couldn't buy your way into heaven. Radical stuff. The Pope was most definitely not amused.
Ulrich Zwingli: came to more or less the same conclusions at more or less the same time as Luther. Without any of the spiritual agonies. Boy, Luther was really pissed off when he got wind of that.
Plus he served with the Swiss mercenaries in the Italian Wars, so he was presumably a bad-ass too.
As radical Reformation theological thinkers go, Zwingli is by far the coolest.
Small catchment, I agree... but still.
9 hours ago