Is the Bible (or the Koran) a work of scripture or of literature? Is it the word of a real, existing deity or the work of humanity? Why should I believe the former? What's the difference between scripture and literature, and how can I tell the difference? Other than by arbitrarily designating some text as scripture and then rationalizing that arbitrary designation.
If it's literature, why should I take the moral, ethical, and spiritual beliefs of millennia-dead, pretechnological murderous slave-owning misogynist authoritarians any more seriously than I take the works of Locke, Paine, Jefferson, Gandhi, or my own reason and moral intuition? Indeed why should I take their beliefs seriously at all, other than as happily long-dead history?
Likewise, is the Catholic church divinely inspired or just a group of human beings doing their best to think about philosophy and ethics? Again: Why should I believe the former? If the latter, why should I take moral or ethical advice from a group of (supposedly) celibate men who do no useful work, have no children or families, and have an obviously personal interest in maintaining their own moral authority and economic power? Why should I take them any more seriously than people who actually make sense? And they wear dresses and funny hats in the bargain: Why should I take these ridiculous buffoons seriously at all?
Much the same (except for the celibate part) goes for other clergy: Why should I take someone seriously at all just because they wear funny clothes, a big beard and have memorized a lot of text to which I give no scriptural authority whatsoever? Why must I believe that moral or ethical advice has to come from a mysterious, unknowable God to be rational and sensible? The notion is ridiculous.
If religious people didn't take their bizarre superstitions as granting them supernatural moral authority, I wouldn't pay the slightest bit of attention to them. If you want to rub blue mud in your navel in the privacy of your own home—or even in a public park for all I care—you'll get no argument from me. If you want to minimize your religion to meaningless ritual, unfalsifiable metaphysics and ethical irrelevance, I'll keep silent.
But people do take their bizarre superstitions as granting them supernatural moral authority. If some ethical belief makes sense to my natural reason and natural moral intuition, no divine provenance is needed. The only reason to invoke God in an ethical context is to justify beliefs contrary to reason.
Science is the study of beliefs that reality is shoving down our throats, whether we like it or not. Religion—even the most moderate, humanistic religion—attempts to use fantasies about God to shove ethical beliefs contrary to natural reason and moral intuition down our throats. I'll take it from reality (what choice do I have?) but I won't take it from you.