I need to correct what I wrote in my last post. Levi's statement has nothing explicit about devilish vs. divine sides of the imagination. It may well be that such a concept is implicit, and that he indeed does look down on Christ, as someone in a permanent state of inflation (until perhaps when it's too late, and he says "Why hast thou forsaken me?), one way of losing one's reason.
But here I think his thought, at least explicitly, is that too great an influx of light, or for too long at a time, can destroy one's ability to reason, reducing one to the state of the madman (Le Mat) or someone whose capacity to reason was very limited in the first place (Le Fou). The analogy is to someone coming from the darkness of Plato's cave into the full light of the sun, but with more of a downside. Here is Plato (Cornford translation,
Republic VII.515):
And suppose someone were to drag him away forcibly up into the steep and rugged ascent and not let him go until he had hauled him out into the sunlight, would he not suffer pain and vexation at such treatment, and, when he had come out into the light, find his eyes so full of its radiance that he could not see a single one of the things that he was now told were real?
Levi, I think, is saying that the situation is worse than that. If someone in such a situation were to look directly at the sun for more than a moment, they might damage their retina and drastically reduce their power to see altogether, i.e. may have even more than a brief period of mindlessness and sink, partially or wholly, into oblivion (including states of some functionality but with fixed delusions, i.e. that they are God). Or he might see for a while and then become blind, perhaps temporarily, as in the case of going into sunlight amplified by snow, blindness, i.e. mindlessness, for a period of time, but if it isn't too long, followed by recovery and memory of what one saw. (The Myth of Er gives Plato's example of such a case, a near-death experience.) In that way, the state of the Madman could be a transition to the state of World-consciousness. Or it could be a fall into the abyss.
Perhaps Levi had heard the story of the four men who studied Kabbalah: one became a heretic, one committed suicide, one went mad, and the fourth was Rabbi Akibba.
Or perhaps he knew people who had experimented with hallucinogens.