I converted FROM christianity because of stuff like this...right now I'm studying another (paganism/wicca) because it fits so perfectly....and I STILL have people (though it's not longer 1692 last time I checked) trying to save my soul, threatening me and calling me a satan worshipper because they're stupid. (the devil is a figure in the christiant faith, we don't even believe in one, let alone worship one :P)
ANYWAY...I'd sit down with your parents (and snakes
) talk to a priest, use SnakeLady's quotes and any other info that you can gather to prove that snakes aren't evil...
while it sounds silly to some, it's bringing it all down to your parent's values and how THEY see things....If I were in the same situation I'd be doing this...
by not trying to make them see that they're being ridiculous-ish (not trying to offend), but by giving them information so that they can get a better idea....it makes for a rational discussion anyway.
Remind them how they're your pets and that you love them very much, that helps
Best of luck,