I'm also 17 and I feel like it's too young to have your parents respect your decision. It comes off as a "phase" sometimes and they may be more slow to accept it as if you were an adult. My immediate family is largely Atheist/Agnostic except for my mother, who knows our "doubts", but we're somewhat careful about flinging the word "Atheist" around, as she thinks they're scum.
Your parents may not be as bad, but it will still be difficult in convincing them. I would say keep rational at all costs, be very logical, and bring up ethical issues in religion if you have to. Convince them, at the very least, that you're a good person.
My advice would to be consistent, delivering a simple, clear message, just as you wrote below that you don't believe in God and don't agree with religion.
If they are faithfully religious it will be hard for them to accept it, and you don't need to drive it home, but every time it comes up, just make sure you stay firm. Reassure them that you are still a good, loving person, who loves them and always will, and that you have good moral structure (assuming you do) not because of "God" or the Bible, but because of the wonderful example they have set for you through how they treat you and the world around them.
Also make sure you keep it as light and loving as possible if you can. That will assure them that you are really OK without religion.
I'm 28 and haven't practiced my parents religion in nearly 12 years, and they still don't believe that I'm an atheist. I've told them that I don't believe in god, nor does my husband, and they still CHOOSE to live in denial. My question is, if you've never actively practiced their religion, then why do they *have* to know that you don't believe? Do they push the subject? Do they treat you as if you're xtian? I know that can be annoying, but if they aren't actively trying to get you to go to church with them, then when it comes to "hopeful" parents, my philosophy is that some things are better left unsaid.