The basic analytical model of intersectionality states that one can have one or multiple minority statuses.
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They wouldn't be a minority for being white though, in this case.
It isn't their whiteness that'd cause them to face discrimination. It'd be their religion.
Just like how a black person can be discriminated for being black while "normalized" or whatever for being simultaneously Christian.
It is how intersectionality happens.
It's possible to be a minority and not a minority at the same time
Considered by whom? Considered in what way(s): legally? Which part(s) of “the West” are we talking about? Some of the questions you ask seem to come loaded with unexpressed presuppositions.
You may already know that in US law, religion is one of the protected classes, which is supposed to provide anti-discrimination protections.
Minority in which sense of the word? That’s way to broad of a descriptor.
In the sense of “Religious minority”, then by definition, yes. This is really a question that relies heavily on context.