I didn't ever really make a distinction between normal swearing (usually about bodily parts, fluids, and actions/professions involving them) and blasphemy, apart from, when talking to Christian friends, feeling a little self-conscious about using "Christ!" as a mere interjection, when to them it's a good bit more important.
The Italians have some wonderful blashpemies, and as a non-native speaker, it's very easy to assimilate them. In the office once, after I came out with "Porco Dio!" (piggy God), larsen suggested that I should maybe be aware of who I was saying it to, as it was a "bestemmia" and therefore in quite a different category from normal cursing. And in fact, a few days later $boss commented unhappily when I said "Porca Madonna!" a little too loudly ("Porca puttana!" - "Piggy whore" - wouldn't have raised an eyelid of course).
Yet I was surprised to find myself feeling something which I think may be similar to the emotion of outrage in the face of blasphemy, on seeing someone in an online forum with a nick of "Muad D'ib". I'm not sure if this is because this prophet of a fictional universe means more to me than one venerated in this one, or whether it's because by and large people don't call themselves "God", "Jesus" (unless they're Spanish speakers), or similar just because they think it sounds cool.