I wouldnt say anything about it. At 16 you are a young adult and now you will be forming your own world view and your going to change it a million times.
Dont completely close your mind from the modern Christian message either because it really has some great instruction that fits everyone.
Take the good things and discard the nonsense because Atheists can be better Christians than Christians.
My version of that is "Dont Be an Asshole"
All the Best.
I think most kids growing up aren't "really" Christians, they just go with it as you did. At some point or another they start to think about it more deeply and stop just going with it and start exploring on their own. That's normal, and a good thing, as long as you keep exploring. Where you are now, as a young atheist, is fine. Your understanding will continue to change with time. Be open to that.
The first rule is that your parents love you, and always want the best for you. Understand that, and love them for that.
The second rule is that your parents won't always understand or accept what's best for you; like all of us, they are a product of their experiences and they want to use those experiences to steer you where they think you need to go. That can create a lot of conflict with teenagers your age and their parents, if you (and they) let it. Understand that, and be patient with it.
I think a good rule is that when we are guests under another's roof, we respect their practices. I'm a Catholic, but this past week as a guest with Jewish friends, I participated in Passover. I have Islamic friends, and am supportive of their practice during Ramadan. I have atheist friends, and when under their roof I am open to and supportive of their philosophy. That's just being a good guest. In the years ahead, you will have friends of all stripes, and be invited to weddings in churches and synagogues and courthouses and fields. You go and participate not because you believe in their church, but because they are your friends.
So sure, while you are at home, continue as a good guest, because you care about your family. Go to church and participate, just as an act of kindness.
At the same time, it's probably not a good thing to be dishonest. Don't pretend to be pious. If at some point if you feel the time is right to let family or friends know you have doubts, that's an OK thing. I don't think it's something you should throw in their face, but if there's a time when it seems right to share that's fine. Gently.
Accept the fact that parents and family will worry about you, and be annoying at times, and it may take time for them to adjust their thinking. Be patient and understanding.
Find good people to talk to. Atheists and religious, both. Build your own network of support. We all need that. Hopefully many here can help, but look especially for "live" friends for your network as well.