The law cannot state which religions are legitimate and cultural, and which are not.
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That is where your logic fails. The people can determine which cultural religions are needed for the understanding of Western Civ.
And which “The People” are those?
You would hope that it would be parents, but it won’t be. It will likely be school boards, for better or worse, mostly worse, and it often boils down to who screams the loudest, continually complains, demands censorship one way or another, etc. Either way it won’t be “responsible adults.”
If you live in a blue area, the leftists most definitely want to micromanage school curricula. The same with strongly Mormon areas. Evangelicals will control in parts of the US, Catholics in others, this, that and the other, elsewhere.