Casanova said on the subject of the "great secret" of Freemasonry, that if you know what it is, don't say anything to anyone, because if they are not smart enough to figure it out themselves, they are not smart enough to understand it even if told by another.

I tend not to think that 'smart' implied pure intellect as it would seem to lean more towards spiritual discernment. I cannot help but be reminded of Dion Fortune's Chapter VI on Hauntings in her occult classic "Psychic Self Defense" where she stated without apology that, "To perceive a 'haunting,' one needs, as a general rule, to be slightly psychic; it is for this reason that children, Celts and the coloured races suffer severely from such interferences, and the stolid Nordic type is comparatively immune."  

As intriguing as I find her observations & perception of racial karma and its astral capacity, the point here that strikes me as being relevant is that there are indeed variations in spiritual perception, sensory skill and discernment (being a natural ability that may be perfected with practice).  Just as there are various kinds of intelligence that one might be born with, there appears to be a great variance in people's ability to resonate with spiritual concepts or experience spiritual phenomena.  As the substance of the Craft of building that house not made with hands is undoubtedly spiritual in essence, to properly build one must be a receptive vessel for Divine Inspiration.  This is not possible with dismissive and narrow outlooks. 

Mystics successful in their life’s journey often come to an epiphany early on in their youth where they realize that the reactionary doubt that can arise in reaction to a notion or tale of the Supernatural shared by a fellow man is something to be subdued as it is the very restraint that keeps a profane man from insight into the little known sciences and realities of the unseen.  The subjacation of such a flaw in the Mystic’s character goes hand in hand with the Masonic Art were we develop the Listening Ear which itself works quite well with the Silent Tongue.