It would indeed be the genetic fallacy to argue that because Christianity has a dubious origin, therefore the religion as a whole is dubious. But I add Occam’s razor (or abductive reasoning) to the history lesson, so the inference isn’t as simple as that fallacy.
The question is whether, given the historical connection to Zoroastrianism, that is, given the evident contingent evolution of monotheism (from polytheism to henoism to monotheism), what’s more likely, that one step along the way of that evolution is the absolute truth or that all the stages in that development are based on flawed metaphors and mental projections? It’s like the evolution of species: is our species the ultimate one even though we’re just another twig in a vast branch of interrelated species?
What I’m targeting there is theistic literalism. Indeed, Christian theological interpretations run the gamut from conservative liberalism and dogmatism to liberal relativism, mysticism, and fictionalism (myths as metaphors). Again, it’s what I call exoteric or literalistic (especially Americanized) theism I’m rejecting.
Paul Tillich is a good existentialist, so that kind of Christianity isn’t really a problem for me. However, liberal Christianity can become postmodern in some problematic ways; it’s hard to trust in a religion that’s your private creation and that’s the result of eclectic cherry-picking. The idea behind religion is surely to identify with something greater than any of us.