Inciting violence is already illegal. Additionally, some protected classes are ideas (religion, usually implemented as being a member of a major religion) rather than intrinsic values (which I believe do deserve some degree of protection).
The protection of religion comes from sectarian violence, but this is better covered by other laws, and disliking a particular spiritual belief is no different from disliking any other belief.
You're free to call me a taig and papist, show Sinead O'Connor ripping up a picture of the pope, or exhibit Serrano's 'Piss Christ'. I'm free to laugh, realise that O'Connor was making an incredibly accurate complaint about the church's abuses, and pay for tickets (Serrano's an excellent photographer, I've not seen his sculpture first hand) accordingly.
