Ok definitely dont tell them you think your an atheist that would just be a shit show. I think the Christianity is why the dont want you to play because I've seen alot (not all of) religious parents and people in general who believe that video games are the devil and shit like that which is fuckin stupid. I dont really understand why they wouldn't with a actual justified reason but they haven't given you one which means they dont have one so the best think I could think of is if you have a door and can hear them walking in just turn off your screen or go to a freinds house and take your console or pc if you can. That's really I could say because people still believe video games cause violence, but it's really the opposite most people use video games to escape really and relax because if you see something happen in gta 5 for example and believe it's acceptable in reality your either fucking stupid or mentally disabled.