God would be fine. Theism does not bother me, but I might ask my teen what they believe and why. Depending on what their answers were, I would gauge what to ask and say. I might forbid them from going to a certain church, and communicate to them how much I dislike certain beliefs, again depends what is bringing on this belief in God. If they wanted to explore Judaism, I am an atheist Jew, I would be fine with that, and Catholicism also, but my kids would know what I believe, and know how much I dislike religious fanaticism.
If they were saying they were interested in being Christian (I am excluding Catholicism for this question, and mentioning Chrisianity because it is the most likely in my country) it would bother me, especially if she was being told, or beginning to believe, Christianity is the only right way. If she were preteen and came out with wanting to be Christian, my assumption would be some Christian is working on her, and that would really piss me off. I would call the other child’s parents and tell them they have crossed a line if I felt like it was going too far. I hate the word forbid, because I think sometimes children do the opposite then, but I have a hard time believing a child of mine would want to go anyway.
A relative of mine used to be very open on these things. When her kids were little she used to say she thinks it’s fine to learn about different religions, and her kids can decide when they are older. She is from Mexico, and for the most part everyone was Catholic in her circles, although half the family is Jewish. I told her once, “I am not as open as you, I don’t want my kids exposed to other religions until they are older.” I would be completely fine with my kids celebrating Christmas with a friend, or having dinner with a Muslim friend on one of their holidays, no problem at all with them playing with friends from every different religion and nationality, but not ok with other children or adults preaching their word, there should be a feeling among their peers that each child is what their parents have raised them to be at such a young age in my opinion. Anyway, this same relative, last I saw her, her high school aged daughter had received some Christian sort of invitation, and this suppossed relative of mine who used to be so open, said something in disgust, sort of making fun of the Christians who do this sort of soliciting. I really think living in America, in a town that is borderline biblebelt, has really changed her tune. Her old views have changed now that she better understand what goes on. She has become much more closed like me. She is a theist and a Catholic.