There's no way to make this analogy that doesn't sound self-laudatory, but nonetheless it's the best way I can think of to explain what I think is going on. What we're doing here is much analogous to the situation in Plato's Allegory of the Cave. (I'll assume you're all familiar with it, though if not I'd be glad to explain.) For the people tied up watching shadows on the cave wall, talk of colors or 3-dimensional objects, or, even worse, sunlight, is simply unintelligible. It's pure nonsense-talk, and people in such a position will insist on trying to explain all intuitions or experiences through the limited vocabulary of cave-shadows.
Meanwhile, people who have explored to some degree the more metaphysically complex world outside the cave will still have some understanding (albeit perhaps a clumsier one) of the logic of the cave dwellers, but they also clearly perceive that there are some things that the cave-dwellers' vocabulary and logic cannot describe or grasp. So when their metaphysically impoverished comrades demand that they explain these other entities in cave-language, this demand simply cannot be met. The logic of this higher realm will not be intelligible to them until they've done some credulous exploration of it on their own. The skeptic will necessarily be stuck in a trap of his own making.
I don't remember hearing before the word "credulous" used with a positive connotations. Credulous is the person who believes the advertisement, who falls prey to the conman, who believes in phrenology or astrology. A defining feature of philosophy is to rise above the credulity of the masses, critically examining one's every belief.
And yet I found myself sympathizing with Joyless Moralist's reversal of the word's usual connotations. For me this is the end of a road which began with Descartes, not literally with reading Meditations on First Philosophy or something but with a recognition of the moral imperative of questioning one's beliefs to the very foundations in order to be honest in one's worldview, which in my own life paralleled to some extent the history of modern philosophy. In The Pilgrim's Regress, C.S. Lewis represents Reason as a virginal warrior woman, who appears in the hero's darkest hour and slays a giant to free him from a hideous prison. It is a moving and inspiring scene, but unfortunately obscure, so I'll substitute a more familiar analogy: the Hebrews walking through the Red Sea. Just as Moses led the Hebrews through the Red Sea on dry land, and then the sea drownd Phaoroh's following army, Reason can liberate us from all manner of superstitions and powerful falsehoods.
But only into the desert of skepticism. The Cartesian project fails in a particular way. I think the cogito ergo sum is wise and infallible as far as it goes, but it doesn't go very far at all. It seems to be the generally accepted view that Descartes' argument fails thereafter. I haven't read it carefully enough to say, myself, and the general outline of the argument-- first prove the existence of God, then rely on a good God not to comprehensively deceive us-- I sort of like, but that Descartes fails to apply the rigor to which he initially seemed to aspire I have little doubt. Hume later carried the Cartesian project (or something like it; to what extent British empiricists like Locke and Berkeley, who were more influential on Hume than Descartes as far as I understand, took themselves to be continuing a project begun by Descartes, I don't know) to its climax with his demonstration of the groundlessness of our belief in inductive reasoning, in causation, and in many other concepts necessary to sanity and common sense. Where does one go from here? Is the Cartesian project a mistake from the beginning? But one cannot-- morally one cannot, and in any case one cannot do so because belief is not wholly voluntary-- go back to the unexamined opinions which preceded it and which it demolished. The lesson I draw from this is, as I think I put it in a discussion with Joyless Moralist, that we must be more "latitudinarian about what we accept as primary evidences." She thought this made nonsense of my claim to be in the tradition of Descartes; and I would agree that it involves a considerable modification of the Cartesian project.
Anyway, the unusual way in which JM uses the word "credulous" in the passage quoted captures pretty well what I've come to think is necessary. People who are in the grip of what they think is a well-grounded and certain worldview, whether it be Mormonism or materialism (which, by the way, Reason quickly disposes of by pointing out that there is no grounds for accepting this impoverished ontology, and plenty of grounds for not accepting it since there is much that it exhibits no tendency to be able to explain), tend to trample on the delicate intuitions which I think are essential clues on the path to truth. Only when the false certainties have been demolished by an encounter with skepticism (symbolized by Descartes) can one realize-- like a shipwreck victim who spares every nail and every bit of cloth-- how valuable these intuitions are, because they are all we have. Reason leads one out of the bondage of socially accepted formulae and superstitions and falsehoods into the desert of skepticism, but I think "credulous exploration" is a good description of part of what is needed to learn truth.