I don't think that arguing against objective morality, or arguing FOR it but towards a naturalistic source, are the only options. Take a look at what happens when we (even hypothetically) concede that it has a theistic basis! It certainly doesn't do Christianity any favors.
Why couldn't an objective morality simply make sense? Why does looking at morality through the lens of Christianity make such LITTLE sense? Well, it's obvious why - Christianity and its god is nonsense. There could be no clearer sign that Christianity is a man-made bunch of bollocks, than to look at its claims on objective morality. All these special pleadings about context and new vs old covenants are the dying gasps of a belief system that before long won't have anyone even TRYING to make it make sense. You'd be better off trying to argue that Joseph Fritzl is the greatest father that the world has ever known (within a certain context).
Look elsewhere for an objective moral law giver, other than Christianity, for the god that you are convinced must exist. It can't hurt - and the bible has caused enough problems on this planet, with its contradictory and wicked claims and commands. Don't support, or give cover, to those who want us to base our modern societies around that absurd mess of a book. Be honest with yourself, and reject it, as people do who take a real, open, honest look at what it says. It isn't divine, it is appallingly man-made. Man-made as in MALE-made, too, quite obviously.
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