There are about 400 theories about that, and all of them seem nonsensical or plausible.
He did have some documented meetings with Crowley, so for sure it played a part. Lots of stuff talks about the Crowley/Le Chiffre/Blofeld link, which suggest maybe Fleming wasn't an occultist, what with making the Crowley-esque characters the bad guys and all.
I don't know what I'm taking about, but I once did some research on this while on the toilet.
