Reflective Dialog on Meaning and Morality

I've just about finished reading Ravi Zacharias' book The Real Face of Atheism. It is a concentration of many ideas he has used in his talks concerning meaning and morality. One element that ties strongly into the question of meaning is Ravi's critique on a naturalist's position that a human mind is the sum of time plus matter, plus chance.
I have been thinking, how might a dialog between an orthodox theist and a presumed postmodern atheist play out?

Theist: Without divinity, there is no meaning.
Atheist: That's preposterous! Existentialists make their own meaning. (consider also Positivists)
Theist: Hmm, so is there any meaning that can be wrong? Let's say the highest virtue should be slicing babies in half.
Atheist: (Just one possible, but quite plausible response) Well that wouldn't be a very proper meaning.
Theist: So, just what defines what is proper?
Atheist: Strictly speaking, nothing is proper. But to indulge the question, let's look at the common denominator that leads to a proper, enlightened moral view, like humanism. In that case, it is a matter of biology that tends to tell us to be benevolent to our neighbors. Its mechanistically predetermined by the nature of our universe.
Theist: Then, you are a prisoner of your impulses? If, like Bertrand Russell, you distinguish right and wrong by "feeling," then aren't you nothing more than an automaton? And what role can reason play where feeling is the basis of decision?
Atheist: Okay. Lets say there is an objective, right-and-wrong divine author of the universe. Then, aren't you a prisoner of circumstance? Aren't you compelled to obey?
Theist: The choice to obey or not is always ours to make. And fortunately for us, God is good, so the circumstance we're imprisoned to is a good one at that.

That to me, seems the logical end of the argument. Toward the end, we see the atheist unable to reconcile the basis of his morality with reason. Reason points nowhere unless you take biological determinism as a lead. Then in this case, you are a prisoner, or automaton. But this cries out against an instinct within the intellectual, who hopes to be critical in his approach, not unthinking. So here we have our good old friend, contradiction. There is no exit from this predicament, unless one concedes an external (objective) purpose and meaning as an escape from determinism. An objective purpose and meaning is basically divine.

If the atheist cannot solve the above conundrum, at least he can spit the question back at his accuser. If divinity made the universe as it is, and we are limited in our power to shape the foundational nature of the universe, then aren't we prisoners of circumstance? Isn't the choice we make between obeying God or not rather a false one? The Christian can answer satisfactorily that God is objectively good, the nature of the universe is objectively good, so if by some reason you deduce that we are prisoners, then it is to a good circumstance. This is nothing to say of the nature of free will, of which many books could be written about, but none by me.