Atheists with philosophical leanings - like the inestimable Ivan - tend to cede ground to theists on the issue of morality. They agree that you can't ground objective morality without a God, that atheism entails a kind of existential despair, understood as a lack of purpose. And this makes sense! Atheists don't have any goals independent of our own desires and wishes.
Let's continue to agree with theists that morality without a God is irrational. But let's go further, too, and claim that objective morality doesn't exist even with a God. Take the best case scenario:
God appears in the sky, accompanied by angels and trumpets, and tells all inhabitants of the world to follow a single moral code, which he communicates clearly and which - through some miracle - nobody misinterprets.
But would even that allow us to claim that rightness or wrongness is a factual matter? Here are some considerations that a rational believer would be forced to make, in the event of a divine visitation.
Am I going crazy? On the balance of evidence, it's probably more likely that I'm having a mental breakdown than I'm actually seeing God. There are a small number of people each year who receive 'divine commands' - on what grounds do I dismiss their personal experience while accepting my own? Now if everyone in the world receives the same visitation, that would seem to rule out mental illness. It wouldn't rule out mass hysteria, though, or the possibility that my individual delusion is severe enough to include the illusion that everyone agrees with me.
Is this figure God? Even if the man in the clouds proclaims his divinity and performs miracles, that's only evidence of some power, not omnipotence. A technologically advanced alien might be able to 'play God' with a high degree of effectiveness.
Finally, and most importantly, why should I listen? David Hume's is/ought distinction has an almost mathematical exactness - there's no possible way to connect facts about the external world with moral obligations, even if those facts are delivered by a deity. If I'm a strict deontologist (a believer in moral rules) and God descends from heaven to say "sorry guys, the utilitarians are right", why should I accept his judgement? Because you're defining God as having knowledge of all things, factual and moral? Well, unless you give me a very good reason to believe in the existence of 'moral facts', I'm going to call that definition of God incoherent. (Of course, if you presuppose the existence of moral facts you can propose a God that would justify all kind of things - like the existence of moral facts.)