OK, a bit of an interest of mine (topping an RE degree with an MA in asd LOL) so I have a few thoughts- however so much varies n your childs age, language skills, etc.
DS1 has AS / HFA and also attends a Church school. I've tried to explain that in any life a lot of events happen, both good and bad, that we cannot rpedecit and some people feel they need to explain that and the way that works for them is religion. However, religions are usually very old and so the exact details of what happened have been lost- looking at Christianity, we know there was definitely a man called Jesus. What we don't know is much about the details; we only have the stories passed down by several people and they don't compare exactly. Plus there is a chance that a game of chinese whispers has happened, or that possibly some peoplefelt a realcompetitive need to make their preacher /faith the best one in a time when wandering Preachers were actually rather common and embellished.
If you believe in a faith, you have to reach your own beleifs about the details of things 9through prayer etc) but it is the message that matters most and most of the major faiths have similar ones- the value of kindness and love,duty towards your family and community, that sort of thing. 'God' an be lost in the details sometimes.
if you don't believe, it is perhaps easiest to understand as a way of explaining thinsg that peoplearescared of.
But either way the things like outfits, meals, exact words- all the things that mean so much to those of us (I have many traits) with a spectrum brain are not absolutes. The religion is in the faith and teching; the rest is the amalgamation of stories acquired from people who lived after Jesus, and we must allow them tehir flaws becuiase they are only human.
Sorry am not sure how much sense that makes- ds1 is fascinated by eligion (as clearly am I LOL) and he has a tendency to focus on the artefacts, rituals etc- I think a spectrumperson can often find a lot of meaning in those. Me, I find it fascinating in the way someone would watch the activities of a bunch of ants- I do have parsonal faith but am interested in the mutiplicites IYSWIM.
At some point I ahve to get ds1 to understand the notion of eprsonal truth, and that is a key in this. A hard one to explain, but useful if you can manage it.