Of course the \'original\' FF7 sephiroth acctually got his name from a Qabalistic text named Sepher Sephiroth. Which was written by Aleister Crowley, a member of the order of the golden dawn amongst others.
He was also one of the darkest and most powerful occultists ever to walk the earth, supposedly. His collection of Qabalistic texts is without parallel in the history of mystical literature. He was notorious for sacrifices, torture, Black Magic, and was the founder of the Silver Star occult. Which is why I would guess they called the big bad Sephiroth.