Why is it some monotheistic religions like Christianity and Islam condemn the worship of many gods, doing witchcraft, and basically most pagan rituals/beliefs/activities?

What you're describing isn't even monotheism, its called mono-lateralism. The latter is a belief system where there ARE many gods, but only one or a select group of them is considered worthy of worship.

As for why the Bible appears to support that idea, I don't like to bring this up here, but its necessary for the discussion. Judaism was once a polytheistic religion. In fact, Judaism developed out of the Canaanite religion. Yes, there's lines in the Bible that support a mono-lateralist system, but these were written after the fanatics I mentioned managed to convince people to only follow Yahweh (who was originally the Israeli war god). Later, the system switched to a monotheistic system, and Yahweh now essentially encompassed the domains of all the previous gods. Nobody knows why this happened, but its possible that the ancient Jews were inspired to do this after hearing about a theory from Greece that all Greek gods were actually one (an idea often called pantheism, which is also believed by many modern Hindus).

So yes, Judaism was once a mono-lateralist system like you say, but it later become the monotheist religion that it and all the religions derived from it are today. NO modern abrahamic religion that I know of preaches a mono-lateralist system. The closest being the few denominations that teach that all other gods are demons.

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