The ultimate grounding of an objective moral system is the set of axioms (or values) it rests on. The values can't be proven, you have to accept them and if you don't you're philosophically justified in rejecting the entire system built on them, since by definition if they aren't valuble to you they aren't values at all as far as you're concerned, but if you do it's at least theoretically possible to build a coherent moral system on them. The more basic you can make the values, the more people the system will apply to. For instance you might start with something like 'people want to survive as long as living isn't too unpleasant', from that you can derive we ought to arrange things so people aren't killed when their lives aren't so miserable they'd rather be dead. There are no doubt a subset of people who wouldn't agree that people want to survive as long as living isn't too unpleasant, and you're never going to have a meeting of the minds on that unless you can persuade them to value something they currently find valueless. I still think it's reasonable to call a system based on an axiom over 99% of humans would accept as at least potentially objective (if the conclusions follow from the premises) and possibly widely applicable, though never absolute.
I'm not anti-Christian. I'm anti-stupid.
