Step 1: Pretend you're religious by smiling and nodding when your family mentions God.
I truly don't have any advice for dealing with family members who insist that your way is wrong and their way is right.
Mr. Atheist suggests coming out to someone in your family who feels the same way that you do, and perhaps speaking with the host ahead of time. If you don't feel comfortable hanging out with your family who tends to shove religiosity down your throat, you can bow out and leave.
For the most part, my younger sister and I are in the same boat. We're not all that religious and find life to be satisfying without following the church's doctrine, but we have different ways of confronting the issue. My younger sister prefers to cut off contact and not show up to family gatherings. I show up and just play along. Sure, I'll hold hands and say grace to keep the peace. I'll even pretend to listen and keep my thoughts to myself when my aunts and uncles talk about the ways God has blessed them by not giving them a job promotion. I'll even respond with "okay" when my mom tells me to say my prayers. On the other hand, my younger sister only talks to my mom when she needs something and otherwise avoids contact with her: no phone calls, no visits.
I've never told my family explicitly that I'm an atheist even though I'm pretty sure my mom knows that I don't believe anymore. She told me she's not sure why I think and believe differently from her when she's the one who raised me. It's disappointing to hear something like that coming from your own mom. I know religion is important to my mom, so I keep my opinions to myself. But somehow I feel like I've still disappointed her. I've learned to live with that disappointment, and I think at this point, my mom has too, or perhaps I'm deluding myself.
I keep telling myself I'll come out after my grandparents pass away because after that, it doesn't matter anymore. For the longest time, I was afraid I would be kicked out of the family because I don't believe. I have avoided phone calls and letters from my grandparents out of guilt that I don't think the same way as they do. Out of guilt or perhaps fear that they'll discover a wolf in sheep's clothes and be very disappointment in me. I feel anxious around my family, especially my grandparents, and I hope they never discover my secret (that I am blatantly putting on the Internet, the least secret place in the entire universe). Last year I had a panic attack in my sleep because I was so worried about going to my grandparents' house in my dreams.
It might be easier to deal with if they shared the same values as me, but we don't share the same values and we seem to be on the opposite side of the spectrum. For now, I'll bide my time and keep my opinions to myself. I'm lucky in the fact that I don't have to live near them anymore so I don't have to hide my true self as much. It feels weird when they say things like, "Oh, we miss you since you're so far away." It feels like they're just saying it to say it and that it couldn't possibly be true since they don't even know the real me.
Family is difficult, and I'm afraid I don't have any answers. I've chosen the route with the least conflict but also with the least satisfying outcome for the relationships with my family. I don't hate my family, and I don't think they're terrible people whom I should cut off. I just wish they would talk less about religion so I didn't have to dread going to their houses for holidays.