Just to make this even more convoluted:
I've found that the simplest answer I can give is that it's my religion. People who have never heard of Thelema still know what a religion is, so that's a simple frame of reference, and the details don't matter unless they're curious and want to go in-depth. It's like if I said I'm a Hindu: people at least know what category that goes in.
But the problem with that answer is, I think "religion" has a lot of connotations for people that aren't necessarily true, and that are derived from Judeo-Christianity (i.e. belief in a Supreme Being). So even if it would be accurate to say "it's my religion", it would create even more confusion and require even more explanation. I think.
Any thoughts? I feel like there has to be a simplistic catch-all answer for casual chit-chatty purposes... but maybe there isn't.