To see why let's define what me mean by "morality". Morality is the study of which choices _ought_ to be made, and which _ought_ not to be. Moral goodness/badness comes from making right/wrong decisions. Therefore morality must be individual because only individuals make choices. Where the statement is made that "the community/nation/government" made a decision we find that in fact individuals made decisions. Even in cases where the decision was made by voting individuals decided how to vote, and those who voted against a decision cannot be held responsible for it. Therefore the concept of "collective" guilt is a nonsense. The only guilt or virtue is individual. Yet the Abrahamic make guilt not only collective, but species wide. We are asked to believe that an "objective" moral standard holds us responsible for actions that took place before our birth. It would be like holding me responsible for the assassination of JFK, over 5 years before I was born. This is the doctrine of "original sin", that punishment is valid for acts which the person could neither cause nor prevent. In the words of Ayn Rand "They call [this] a morality of mercy and a doctrine of love for man.".
But to judge the Abrahamic religions only for how they think about sin is to miss half their depravity. How they think about forgiveness is, if anything worse. Having designated the mere fact of our existence as a crime worthy of eternal pain* they offer a way to be forgiven. Note that the possibility that one might be _worthy_ of forgiveness is denied us. Our moral status cannot be changed by our own acts, but only by the decision of God to forgive us. So not only is guilt the result of someone else's actions, so is forgiveness. A morality based on nothing you do is not objective, it's not morality and it's not worth considering.
So what we're advised to do is beg for a forgiveness that we don't deserve. Note what is being suggested, to try to gain something without deserving it. This is by definition not moral. The definition of something not deserved is something it is not moral to choose to gain, at least not until some action can be taken to deserve it. The method of gaining this undeserved thing is begging and obedience. Yet begging cannot change whether something is deserved. Begging is an attempt to convince someone of the rightness of an action, not through demonstrating it is right for them, or morally right, but beneficial for someone who deserve the benefit but is pitiable. The justification for us being forgiven is that someone else was punished for our sins. But since moral status is individual, the punishment must logically be individual as well. Since rightful punishment must, by definition a response to a particular moral status, punishing someone for someone else's crime is incoherent. The moral status of one cannot depend on the actions of another.
So if objective morality cannot come from the Abrahamic religions, can it exist at all? Well yes, and it's simply proved. Christopher Hitchens said: "I would submit that the doctrine of vicarious redemption by human sacrifice is utterly immoral. I might if I wished … say, “look, you’re in debt, I’ve just made a lot of money out of a God-bashing book, I’ll pay your debts for you” … I could say, if I really loved someone who’d been sentenced to prison, “if I could find a way of serving your sentence, I’d do it” … I could do what Sydney Carton does in A Tale of Two Cities … “I’ll take your place on the scaffold,” but I can’t take away your responsibilities, I can’t forgive what you did, I can’t say you didn’t do it, I can’t make you washed clean. The name for that in primitive Middle Eastern society was scapegoating. You pile the sins of the tribe on a goat and you drive that goat into the desert to die of thirst and hunger; and you think you’ve taken away the sins of the tribe: a positively immoral doctrine that abolishes the concept of personal responsibility upon which all ethics and all morality must depend.”. This is clearly an objective moral truth, which means that objective moral values are possible. They're just not possible with the false claims of the mainstream religions.
So is an objective morality compatible with a God? I can't see why not, if there is a non-contradictory definition of God. However it doesn't require it. By definition objective morality depends on the evidence available. If there absolute morality depended on a God, and God wasn't objectively proven, then it wouldn't be objective morality. It would merely be the morality that people who think God existed think is objectively true. But there is no reason to believe that an objective morality does depend on God.
* By some interpretations it's only eternal oblivion and denial of eternal life. Personally I think eternal life would be a curse but oh well...