The minimum we can ask of any belief is that it be at least consistent and sustainable on its own terms.
Of course, those here who embrace one or another existential rendition of objective morality are no less subjectivists. They are individual subjects who lived particular lives that put them in contact with one or another God or religious path. And on this path they experienced the comfort and the consolation of having found moral Commandments that "in their heads" they are convinced will bring them both immortality and salvation.
It's a belief that is "consistent and sustainable on its own terms" because they either define all the terms themselves or provide you with a Scripture that they claim is as far as anyone need go in demonstrating the existence of their God.
Subjectivism is not. It can't even clear the first hurdle of rational explanation. That should concern subjectivists: they can know for certain they're wrong, even if they don't consider such alternatives as moral objectivism or moral nihilism at all.
Of course, this being the Ethical Theory forum, they almost never deem it necessary to bring their conclusions down out of the spiritual clouds. Rational or irrational, moral or immoral in regard to what particular behaviors?
After all, the bottom line here is often Judgment Day. One or another rendition of Heaven and Hell. So, it's not enough to defend God and religion up in those spiritual clouds. Instead, most denominations have their own set of moral prescriptions and proscriptions. Do this and God approves. Do that and God doesn't.
But with all of these...
https://en.wikipedia.org/wiki/List_of_r ... traditions
...folks claiming to be the One True Path, how on Earth do mere mortals go about pinning down the real deal? If there even is a real deal at all.