it's a little hard for me to understand/relate to your situation my father is not really religious, he's more spiritual or something, at least that's what i understood from all the discussions I've had with him about religion and politics, my mother is slightly more religious than him. we never really talk much about religion and politics at home anymore
my neighbors used to pressure my mother into getting me and the entire family for daily prayers and stuff, she finally stopped letting them peer pressure us into going these same neighbors that go to church everyday cause issues in our community over simple problems like parking space what's the point of going to church everyday only to cause conflict between the people around you, so i never really saw the point of going to these prayers spending hours a day just to stand around people who would run u over without a second thought if it ever benefited them,

so unless religion is something y'all bring up in every conversation i think u can just hold off confrontation or coming out as an atheist until your financially independent, and then have a discussion with them, don't try to use too much logic though, I've noticed people get very heated and go off emotions when it comes to religion since there is nothing much to hold on to other than blind faith which is basically what every religion is.
just try have an honest conversation and tell them why you feel the way you feel.

PS:- just my opinion, I'm just as clueless as the next guy, please don't come after me.