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 Summa is a book of objections.
The structure of the Summa Theologiae — the central work of Catholic theology, studied for seven centuries — is not a list of conclusions handed down from above. It is a series of quaestiones, each of which begins with the objections to the position the author will eventually defend, stated in their strongest form. It seems that God does not exist, because… The Catholic tradition's flagship intellectual document opens, repeatedly and on every important question, with the case against itself.
This is not an accident of style. It is a methodological commitment. The truth, on the scholastic understanding, is reached through disputatio — through the testing of positions against their strongest objections, the canvassing of authorities, the exposure of one's own conclusions to the most rigorous available challenge. A medieval theology student was required to argue the case against Catholic doctrine before he was permitted to argue for it. The intellectual culture this produced was, by the standards of any age, unusually adversarial and unusually rigorous.
The objection mistakes catechetical instruction — which is appropriate for children learning the basics of the faith — for the entirety of the Catholic intellectual tradition. The catechism is the floor, not the ceiling. The ceiling is six centuries of scholasticism, the modern Catholic philosophical revival, and the ongoing magisterial dialogue with science, philosophy, and culture. Anyone who has spent ten minutes in the actual literature knows this.