Exactly. The fact that it was, in this case, religioulsy based is not really the point - thier motive, yes - but not really the point.
What if they had offered to take this child to a rock concert. The OP thinks her DD is too young to go to a rock concert and says 'no'. The story plays out the same way - with the DD over at their house with the OP having no idea she had even left the yard and the other mother calling to say rock concerts are no big deal.
I think everyone would agree that the other child and the other mother were out of line, even scary people, for doing what they had done.
Now, having said that, I think the OPs DD needs a good talking to about walking away from home without a word. If my DD had done that she would be grounded for sure and other punishments pending the seriousness of the crime.
In this case, people the OP didn't even know well? Very harsh punishment indeed.