Is your boss someone that you could talk to easily? Or at least without extreme amounts of difficulty? If it were me, I'd go to my boss, in person, not through e-mail, and explain to him, non-confrontationally, why you really do not want to do that, and kindly request that he appoints the task to someone else. I find that people in general, even bosses, are more likely to be agreeable in person. Through e-mail it's too easy to tell someone off because you're not looking at someone face to face.
In my experience, using words like sin, Catholic, hell, etc, are words that offend people like your boss. If you have a family depending on your income I would try to be a discreet as possible. Try not to offend at the moment, and do what Stubborn said- look for another job.