[gingerrr]

His arguments against absolute open-mindedness weren't grounded in discussions of his faith - even just the full version of his quote shows that, it was in reply to a metaphor for theory of mind his friend told him:

"For my friend said that he opened his intellect as the sun opens the fans of a palm tree, opening for opening's sake, opening infinitely for ever. But I said that I opened my intellect as I opened my mouth, in order to shut it again on something solid. I was doing it at the moment. And as I truly pointed out, it would look uncommonly silly if I went on opening my mouth infinitely, for ever and ever."

Other variations of the quote he told included an addendum: "The object of opening the mind, as of opening the mouth, is to shut it again on something solid. Otherwise it is more akin to a sewer, taking in all things equally."

Neither of those relate to grounding his epistemology in Catholicism.

While I definitely agree his Christian apologia means a heavy dose of salt with intellectual claims, he's not even arguing for a moral center in this metaphor. Simply an intellectual self, to not be a gaping maw of consumption, as you wouldn't be physically.

Discernment with respect to ideas you're willing to entertain isn't exclusively the domain of religion, as far as I'm aware.