I’m inspired by this comment thread to finally prove something I’ve stated before: if a person fails to understand an idea after a certain amount of time and tutelage, it indicates either low general intelligence or immorality. If a person observably has high intelligence, then they must be suspected of immorality. There may also be ideas that are impossible to grasp without sufficient intelligence, perhaps due to sheer capacity or perhaps low learning speed and insufficient time (for instance, a person with 50 IQ could study math for a lifetime and never reach Fourier analysis).
I’ve previously defined intelligence as a generalized ability to surmount nonmaterial obstacles in pursuit of objects of desire. Given sufficient time and ability and desire, the obstacles will be overcome and the desire satisfied. If a preponderance of time and ability does not produce the object, then we must conclude that the desire was insufficient or did not exist. Because the strength of desire for truth is a virtue (in the Aristotelian sense), and Jesus is the Truth, then deficiency in this virtue is immoral.
In nitty gritty terms, sometimes people don’t understand because they don’t want to understand.
It also seems clear that certain sorts of minds can be artificially limited by a worldview. Math-y materialists who excel in formulaic application of rules have trouble understanding the philosophy that underlies their work until their delusional materialism is destroyed by Descartes’s cogito argument. Their tendency to materialism is likely due to their aversion to holding judgment in abeyance (local incoherence leading to dissonance). The mind-body problem, though absolutely necessitated by the existence of both external sense data and the cogito, is perceived as repulsive to such people because it is unsolved and possibly insoluble. This sort of phenomenon may be explained as low general intelligence, though the particular sort of intelligence (mathematical) may be quite high.
Edit: Inspired by an earlier comment by PhillipGeorge, I’m going to name the above form of immorality “foolishness”, and use this proof as the definition. It applies perfectly to the biblical usage.