I'm leaning towards these:
Nour: Practicing doctors, heart surgeons and engineers may sit there telling you how the ginn stole their wifes jewels
Bobonaut: Hellos and welcome dude...
IMHO, you should never tell them at all. Why make them live with the pain of having an atheist son, they won't get it anyways.. i personally never told my parents, just not to break my moms heart..
you know if you where a christian who decided to be muslim or vice versa, you will have some trouble as you have rituals to attend to and stuff, but you are not..
Just live your life and enjoy it and find ways of keeping everything for yourself..
Keep it to yourself until you are not financially dependent upon them. The reality is that you're still a kid and you still need them for support. I think that once you have moved out, have your independence and can look after yourself then you can look into telling them. I think it's important to tell them, but not required as of yet.
There came a time when the risk to remain tight in the bud was more painful than the risk it took to blossom. ~Anaïs Nin