While monotheists call the Decalogue a "good starting point for morality," they seem to forget that the first four or five commandments (depending on which version you're using) are strictly religious rules. If your religion isn't one that recognizes the Decalogue, then the Decalogue relegates your faith to the back seat of an abandoned car in a locked garage in a bad part of town.
And once you start dropping the commandments that don't necessarily apply, how many are you left with? Six? Three? One?
If we're going to have a common moral basis, it's hard to go wrong with the Ethic of Reciprocity. Christians call it the Golden Rule, although I'd argue that it should be balanced with the Silver Rule, the Iron Rule, and the Platinum Rule. It's perfectly possible to build a moral system using the EoR and not involve Deity of any sort.
Drawing from my own experience, I'm really not impressed if someone is religious or not. And I am certainly not impressed with which religion has "contributed" most. It's what an individual chooses to say and do, not the label. The label has no vice or virtue, no worth except what the individuals brings.
I've known amazing atheists and terrible Christians. Great Muslims and lousy Jews. Worthy Satanists and despicable pagans. Religion can free your mind or impose tyranny. It's the individual that makes that happen.
If we don't share a common or related faith, we can't use our belief to govern the words and actions of another. I have a completely different idea of the afterlife than you do, and it has very little to do with reward/punishment. Nor am I the only one with differing beliefs.
What common ground do we share then?