Why do Christians/Muslims/Jews worship the Demiurge as the One God? Is not the Demiurge the Great Divider?

Of all of the teachings that are related or loosely related to Christianity and the other 'Abrahamic faiths,' this Gnostic concept, which seems to be one of the central ideas in Gnosticism, makes the most sense of it all, taking into consideration the real world circumstances we face daily, and have faced throughout known history. Their concept of a fundamentally flawed process of creation, being employed by a malevolent or at least inferior god goes some way toward making sense of how so much in life is so problematic, painful, and screwed up, or at least is a slightly more realistic alternative to the more orthodox teachings of Christianity. Metaphorically, the Tree of Knowledge of Good and Evil is central to the Garden of Eden - it was "in the middle of the garden," Genesis 3:3. It was not out on the fringes somewhere. The implication of the story is that it was inevitable that man would "eat of its fruit" and that what this results in is the problem of our perception of duality (good and evil, me and not me, naked and clothed, male and female, etc.). This existence we find ourselves in, or at the very least, mankind's perception of it, is inherently dualistic. How can any being be "all-powerful" or "all-anything," or "One," if there is another being that opposes him, or if he makes a practice of separating things, as you have rightly said?