I would argue that Quincy Harker DID believe very strongly in one thing - his crusade against Dracula. Lifelong obsessions can have just as strong a hold on the brain as a strong belief in religion - if he believed, very strongly, not in the intrinsic power of the cross as a holy symbol, but that 'they say crosses hurt vampires, and I REALLY REALLY want to hurt you, Dracula, so when I hold it up, this things gonna HURT you, or I'll know the reason why.'
I suppose you're right - I don't know anything about Taj's background. Still, it would be kickASS to run across, say, a Jewish vampire hunter who used the Star of David, or a Wiccan one who used the Antlers of Cernunnos, or something. (What would be REALLY cool would be if there were a sort of international headquarters for vampire hunters that was patterned all over with every conceivable religious symbol ever created worldwide, dating back to caveman days. No WAY any vampire is gettin' in there!)
Also, just to clarify - Wolvie didn't cross his fingers, he crossed his CLAWS. He went 'SNIKT', then crossed 'em in a crucifix-shape. Dracula sneered at this and knocked him out, with the claws still extended, then Kurt took hold of his friend's arms and manipulated them so that the claws made a cross again. It was considerably more badass than a little finger-cross.