[etceterae]

You're your parents' child, no matter what happens. So tell them this (or something similar): "You and I both believe in God, though we are in different sects. But will you let any difference between our sects get into the way of our blood relationship? I love you, and I love God. So perhaps our interpretations of the Bible are different, but the fact that I am your child will always stay the same, and I would not like our slightly different beliefs get in the way of that."

Additionally, you're an adult now, and even though you're still your parents' children, the point is, you're an adult. You're capable of making your own decisions of what you believe in. It's not necessary for them to approve of your religion (though it admittedly makes things easier). Talk to them, plan out what to say carefully beforehand, and be firm but not demanding. Reiterate that you love them, and that you're their child.

Good luck with your parents; I hope things work out well. Religion can be both a difficult and wonderful thing.