Let's review:

"Some SCAdians I've encountered seem to take their membership a little too seriously, but is that the essence of the organization, that it's a religious thing rather than a historical study?"

Seems that the answer is "no", although the extent to which some members are described as having identified with their characters might be considered a sort of fanaticism, religious or not. But I can see where SCA members of the pagan or wicca persuasion (and others too) would not want a prospective employer to be influenced in his/her decision by prejudice or by lack of knowledge of what the SCA is all about.

But have we settled the question of what the SCA is all about? (after two years of thread discussion!) "Historical study" doesn't seem to cover it.