In one of the books onm Hubbard (I forget which) they show the form of the cross used by Aleister Crowley on a design he made – a Tarot deck or something. I’m pretty sure it was Crowley’s own design, and not a Golden Dawn thing. The Scientolog Cross is a dead ringer, with its florid ends and its four subsidiary “sunbeams” coming out between the arms of the cross. It looks like Hubbard just “lifted” Crowley’s design.
Hubbard was supposed to be a big Crowley follower back inthe 40s when he lived with Jack Parsons, and there’s a famous letter from Crowley roundly condemning Hubbard and Parsons by name (It has to be a mark of especial honor to be condemned by name by Aleister Crowley himself). As has been said above, Hubbard plagiarized from a wide variety of sources. He certainly took other things from Crowley, so why not this symbol. Crowley himself was no Christian, so Hubbard presumably had Crowley’s thelemic views in mind when he took the symbol, rather than Christianity. But I have little doubt that he adopted the symbol simply because the uninitiated would assume it was a CVhristuian symbol, and made his bunch look more like a religion.