I believe the "French Acquaintance" is at once both right and wrong about human nature. Contrary to nearly every anti-religious person that ever was, it is an observable empirical fact that some, even if not many, devoutly religious people are accomplished critical thinkers.
We have even seen a few such birds here on RF at times.
At the same time, most devoutly religious people are not accomplished critical thinkers. But to place that in perspective, neither are most people in general -- very much including most anti-religionists.
It pretty much takes extensive training, practice, and discipline in not one, but several means of analyzing ideas to become an accomplished critical thinker. For example, most people have at least heard of some strange thing called "logic", but how many people have heard or appreciate the power of something occasionally called, "analytic philosophy"? It's nearly a whole field unto itself, and I can guarantee, if you make a thorough and rigorous enough study of it, you will never again look at ideas in quite the same way.
Do you want to improve your critical thinking skills while yet looking cool? Simply take some university courses in the sciences, especially the "harder" sciences, or in philosophy. But for absolute best results take a combination of courses in both the sciences and in philosophy.
As an aside, do you know why the "harder" sciences, like physics, chemistry, and biology? It's not because they are more prone to arrive at truths, but because they are less prone to arrive at BS. And that has little enough to do with the intellectual qualities of the scientists themselves, and much more to do with how easy it is to make mistakes when you deal with far more complex realities than do the harder sciences.
I contend that a top physicist and a top sociologist are mostly about equally rational.