Religion suppresses critical thinking.
The objection, in full
Religion teaches you to defer to authority — Scripture, tradition, the magisterium — rather than to think for yourself. It rewards belief and punishes doubt. The catechism is a list of conclusions to be memorized, not arguments to be tested. A worldview that demands submission of the intellect is incompatible with the intellectual maturity that defines an adult.
Dare to know! Have courage to use your own reason — that is the motto of enlightenment.
or, in plain terms —Religion teaches you to obey instead of to think.
The free man and the obedient man are the same man.
The objection assumes a particular picture of the autonomous adult: the one who derives every conviction from his own resources, defers to no authority, and treats inherited wisdom as suspicion-by-default. This is a recent picture, and it has not aged well. The man who genuinely tries to live this way ends up either reinventing the wheel — laboriously deriving conclusions his ancestors had refined over millennia — or, more commonly, simply absorbing the prejudices of his immediate social class while flattering himself that he reasoned them out.
There is a real form of intellectual submission that is incompatible with adulthood: the surrender of judgment to authority that has not earned it. The Church has produced this from time to time, as has every institution of any age. But there is also a form of intellectual reception — sitting at the feet of the saints and the doctors, reading them with care, allowing oneself to be taught — which is not infantile but the precondition of any real thinking. I read it; I argued with it; eventually it argued me down. This is how minds grow.
The truly infantile position is the one that says, I will accept nothing I have not personally derived from first principles. No adult lives this way about anything. The Catholic tradition is unusual not in expecting reception of inherited wisdom but in being honest about the expectation. The atheist receives his inheritance too. He just calls it common sense.