In the Middle Ages, I'm not sure atheism would be too much more rational than theism, in any sense. To the average European in the year 1000, being an atheist would probably be about as rational as being a heliocentrist, i.e., not at all. We know all the arguments in favor of atheism and heliocentrism, but they didn't. No amount of rationalism is going to let you judge things based on evidence you don't know about.

The average person back then could probably have given you plenty of evidence for God's existence. The evidence would be weak by modern standards, but not by medieval standards. No one was conducting scientific studies then: almost any assertion not directly checkable was supported by pretty weak evidence. Theism might make few predictions and test them rarely, but the same was true of all the alternatives. On the other hand, theism at least had coherent and consistent answers to a slew of basic questions like "How did life arise?", which atheism did not.

So I think the answer is that the only rational principle that would have allowed you to deconvert in medieval times would be "single-handedly reconstructing modern science".