>Was Aleister Crowley an atheist?
No. Crowley's belief system, Thelema, has been described by scholars as a religion, and more specifically as both a new religious movement, and as a "magico-religious doctrine". But then again most religions incorporate magic: walking on water, flying horses, translation stones, etc.
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>Can wizards be atheists?
Atheism is not being convinced by the claims of theism. That's it. This says nothing about one's beliefs regarding other concepts.
For example, personally, I don't believe magic exists since there is zero evidence for it, except when taking Arthur C Clarke's perspective:
“Any sufficiently advanced technology is indistinguishable from magic.”
On the other hand, there are people calling themselves "atheist witches" and some write books, like Godless Magick: A brief guide on atheistic witchcraft
Speaking of books, consider that J.K. Rowling writes about magic without mention of, or indicating a need for, gods.
It's complicated because the ritual occult practices can be mutually exclusive of belief in God. A typical Chaote argument is that we're all wrong because reality isn't as simplistic as we are... but that our belief is the fuel.
And so, yeah there's some atheists out there, and some theists, who practice "witchcraft".
Then there's the more new-age popular "witchy" stuff like Tarot and Astrology (that I think you're talking about). They tend to all make implicit claims about the nature of oneself or the order of the universe, especially "magick" where you directly ask a divine being for intercession! I would argue that a so-called atheist who takes those seriously is really an agnostic jumping on the atheist bandwagon. Does that make sense?
When I dipped my foot into witchcraft, that was when I concluded atheism was incorrect. If I had stayed atheist, I might have gotten into one of the less claim-heavy, more-methodical schools like Chaos Magick.
That said, there's stuff like this, which seems to contradict me anyway.