Send them a text.

Seems to be the youngsters way of delivering bad news these days.....

Why tell them?

When I was in my teens I asked my mom "Why did we never go to church or anything like that?" She answered "There is no God". I suppose the direct approach works best. Thanks mom for being honest.

I guess it depends on the parents. For me, I just mentioned it during a conversation I was having. My father had no problem with it (my mother had already passed away).

Depends whether your parents are lunatics, and if you're dependent on them. If so, suck it up until you are on your own, and only tell them if they ask after you leave.

With them both sitting down and all dangerous weapons put away ?

Can't say from experience because I've never done it. I just stopped participating in celebrating holidays, and practicing the religion. But I don't think it's anything you can really sugarcoat. If you try and get around the cold truth by like saying you're having doubts or questioning some of the things you've learned or were raised with, they will probably either try and teach you all over again, send you to a clergyman or never leave you alone trying to answer your questions and make you a believer again. Depending on how religious they are it's probably no different than telling them you're gay. A hard conversation to have, no doubt. Any reason you have to tell them? Like I said I've never said it to my parents and while they are disappointed that I don't practice or celebrate, they don't get on me about it. If you're actually in the quandary and not just asking, I hope it goes well.

Let it happen organically but be prepared for how you will say what you need to say. That's how I did it.

Be honest.