As soon as my child was old enough to understand, I would tell them that there are are a lot of stupid people who believe this thing called "religion," where they think a supreme being called "God" controls their life. I would tell them that it isn't true, and people only believe it so they can be forgiven whenever they do something wrong, and that there's this fictional place called "heaven" where people think they will go after they die.
If my child wanted to join a religion, I would let them, but tell not to talk about it with me, or mention it at home in any way.
If my child wanted to join a religion, I would let them, but tell not to talk about it with me, or mention it at home in any way.