Within the next year I am going to be looking into adopting from the foster care system here in Massachusetts... although I don't believe in God, many of these kids are from deeply religious families and instead of trying to persuade them to my own thinking (I'm not AGAINST religion, I just don't believe in God), I'd much rather expose them to a variety of belief systems until they feel comfortable choosing one that suits them. Now, how the heck do I do that?
My experience with 2 daughters 42&23 does not include the great error of sampling religions equal to the opposite Atheism "choice" .... Teaching critical thinking applies to everyday life not just philosophy or Sunday morning habits of other people. .....My adult children are 2nd generation Atheists Courtney a librarian and Darrow an engineer. ....If I were to co-parent/adopt a son with my future spouse my hope would be of co-journaling co-authoring co-creating our story with our son inspiring others upon publishing our discoveries and wisdom that grew within our planned family that did not come from our DNA and loins
I took a philosophy class that sent us out to different religious services, 6 of them, and then had us write a paper. I loved the experience and highly recommend it. I can see the same kind of thing being helpful for kids--only with more adult supervision and questions answered etc. And maybe not a term paper. But you never know.
I would also stock the bookshelves or haunt the library for scripture and literature from around the world, and try to spend equal time and emphasis on Bible, Quran, Gita, Tao de ching, Torah, etc. Let them self direct if they are mature and interested enough; if not provide some structure like "this week we are doing Judaism; next week is Shinto" e.g. Show them it's a big world and there are many very different ways of thinking and believing.
If they've already got their mind made up and aren't interested, I don't know that there's much you should do besides support them in what they choose while unobtrusivesly providing access to info about other systems. Maybe offer incentives for learning about other religions?
Anyway the important thing is you're doing the best thing by adopting so hooray for you!! Best of luck and thanks for being a cool human.