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but not everyone who is not Christian is pagan. There are other religions such as Bhuddism, Judaism, Islam, etc. etc.

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Pagan =/= someone who isn't practicing an established religion. Within a Christian context, everyone who isn't a Christian or a Jew is pagan.

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It's a Christian though that Non-christians were pagan, and pagans were devil worshipers, but neither is true.

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Pagan comes from a Latin term for countryfolk, rural peoples. The religion of the rural peoples was not Christianity (not initially All of Europe was eventually Christianized, obviously), it was indigenous, European paganism. To the Christians, non-Christian was pagan. Hence, by extension, Chinese folk religions = pagan. Buddhism = pagan. Hinduism = pagan. Any other conceivable belief system = pagan.