There's a phrase you'll hear sometimes in Thelema: "The method of science, the aim of religion." This might appear at first to be absurd, but if you consider religion to be the process whereby we obtain genuine and personal spiritual experiences it makes perfect sense. You experiment, you keep records, you evaluate your results and seek to have the same kinds of experiences. I think that this attitude, when it's present, is what separates Thelema apart from other religions.
It is doctrinaire philosophy, and regardless of any benefits it may bestow on you, on the whole it is just as dangerous to thought, reason, and humanity itself as any other system which undermines rationally based systems of deriving belief.
Sorry, I asked for justification and hoped for better. Christianity and Islam also help lots of people, but on the whole they are designed to erode reason and instill faith, and in doing so are net destroyers of human culture and progress.
Please bear in mind that the Temple of Thelema (where I assume you found the quote) is one of several Thelemic groups, and do not necessarily speak for everyone. Still I am genuinely curious as to what you find offensive about that particular statement. If you take the Great Work as the perfection of the individual (which I do) I don't see what could possibly be wrong with that.
If you really think that Thelema is designed to 'erode the reason and instill faith' you are mistaken, but you can't be faulted for thinking that for a brief internet search. Any philosophy is complex, and the things that you're most likely to see first are the more sensational aspects, and those do appear superstitious.
As far as justification for my epistemology I didn't offer that because it doesn't exist. Thelema is not the basis of my epistemology, my experience is. Thelema is how I describe and relate to that experience.
In any case, I don't really hope to or desire to convince you - that's not particularly important. Just hoped to express myself a little more clearly, have some dialogue, and try to understand what you find offensive about the quote.
I read enough on that site to make it quite clear that the Great Work is the actual book they seem to worship.
Regardless, this still seems rather religious, even if some sects are less so. By reinterpreting "great work" it sounds like so many rationalizations of biblical belief.
From: http://en.wikipedia.org/wiki/Great_Work
"The Great Work is, before all things, the creation of man by himself, that is to say, the full and entire conquest of his faculties and his future;"
How you do it doesn't matter. Thelema uses a lot of fancy language because it's Edwardian and its founder was a poet.
It's best understood in historical context. The purpose of Thelema was to free post-Victorian society from the social and moral superstitions and fixations which prevented people from really being themselves. It was essentially the first stirrings of the movement which found full expression sixty years later in the "New Age". Crowley referred to it as a "New Aeon"; poetry strikes again.
The essential tenet of freeing oneself from mistaken ideas, choosing a life goal and doing it, is as valuable today as it was then, but you don't have to call yourself a Thelemite to do it. To a great extent, that "meme" is now mainstream.
perhaps a place to start: https://en.wikipedia.org/wiki/Hermetica
"The study of this Book is forbidden. It is wise to destroy this copy after the first reading.
Whosoever disregards this does so at his own risk and peril. These are most dire.
Those who discuss the contents of this Book are to be shunned by all, as centres of pestilence.
All questions of the Law are to be decided only by appeal to my writings, each for himself."
Thelema and Neo-Paganism in general tend towards postmodernism in their approach. They are generally extremely non-dogmatic and individualistic, and as such they attract scientific types, because they encourage exploration.
What I appreciate most is the fact that humour is at the center, especially with Discordianism. And this is what I enjoy: mysticism but without the serious undertone, and without the demand to enforce the views upon others.
And there is also "humour by proxy" when every so often you share these ideas with someone who follows "serious mysticism" and claims "you are following a satanic ideology!!!" fnord.