"What's wrong with that?"

Political context. Rationalism was associated with atheism, which, for the first time in European history, started making visible inroads into the intellectual class. If you can solve all your problems using your reason, do you really need a God? And plenty of French philosophers hinted that the answer could be "no".

It wasn't just a religious question. Atheism or suspicion of thereof was seen as politically subversive, in the age when most ruling feudal dynasties still relied on God's grace as the ultimate fount of their power - at least in their eyes of the subjects. (But it wasn't always that cynical, plenty of the rulers themselves were quite pious.)