Unless you’re talking about a specific school who’ve heisted the name, Montessori education is not religiously based.
By the time I was twelve, I had discovered that my father (never occurred to me, until now, that it might have been my mother) bought (or subscribed to) a digest-sized magazine called Sexology. There were drawings and text explaining the drawings. That was sufficient to keep me from making other discoveries.
Until I happened upon a large stack of nudist magazines…