I think Lewis knew a great deal about the occult and opposed it. Read his trilogy ending in “That Hideous Strength” which book is all about the occult. People seem to be mistaking a Christian writer for an occult writer because he has the courage to take on the occult head-on. On this one I’ll say that there is some real ignorance and “holier than thou” kind of folks trying to attack Lewis.
As you mentioned, there is that passge in That Hideous Strength:
Would have attracted him once......Suddenly, like a thing that leaped to him across infinite distances with the speed of light, desire (salt, black, ravenous, unanswerable desire) took him by the throat. The merest hint will convey to those who have felt it the quality of the emotion which now shook him, like a dog shaking a rat; for others, no description perhaps will avail. Many writers speak of it in terms of lust: a description admirably illuminating from within, totally misleading from without. It has nothing to do with the body. But it is in two respects like lust as lust shows itself to be in the deepest and darkest vault of its labryinthine house. For like lust, it disenchants the whole universe. Everything else that Mark had ever felt -- love, ambition, hunger, lust itself -- appeared to have been mere milk and water, toys for children, not worth one throb of the nerves. The infinite attraction of this dark thing sucked all other passions into itself: the rest of the world appeared blenched, etiolated, insipid, a world of white marriages and white masses, dishes without salt, gambling for counters. He could not now think of Jane except in terms of appetite: and appetite here made no appeal. That serpent, faced with the true dragon, became a fangless worm. But it was like lust in another respect also. It is idle to point out to the perverted man the horror of his perversion: while the fierce fit is on, that horror is the very spice of his craving. It is ugliness itself that becomes, in the end, the goal of his lechery: beauty has long since grown too weak a stimulant. And so it was here. These creatures of which Frost had spoken -- and he did not doubt now that they were locally present with him in the cell -- breathed death on the human race and on all joy. Not despite this but because of this, the terrible gravitation sucked and tugged and fascinated him towards them. Never before had he known the fruitful strength of the movement opposite to Nature which now had him in its grip; the impulse to reverse all reluctances and to draw every circle anti-clockwise. The meaning of certain pictures, of Frost's talk about "objectivity," of the things done by witches in old times, became clear to him. The image of Wither's face rose to his memory; and this time he did not merely loathe it. He noted, with shuddering satisfaction, the signs it bore of a shared experience between them. Wither also knew; Wither understood...
Somewhere Lewis wrote that "I will not engage in futile phillipics against enemies I have not met in battle. ('...this means then, that all the other vices you have written about...') Well, yes it does, and more's the pity; but it's not to our point at the moment."
His description of the temptation to the occult sounds very much like it was written out of experience: he was remembering, not describing.
Cheers!
Williams became a devout Christian, but in his youth had actual experience with the occult; such practices were common in Britain in the early 20th Century. Arthur Connan Doyle also dabbled in the occult.
I find Williams' All Hallows Eve the scariest book I ever read. The scene where the magician creates a body for two spirits to inhabit is terrifying and it is written in such a way that I wonder whether Williams experienced something like it.
To be clear, Williams is certainly NOT endorsing such practices. He warns against them.