It seemed to me that there was at least one more voice that needed to be heard, that of Christian - and why not, atheist - evolutionists and also a specialist in anthropology and sociology wouldn't have gone amiss either! I guess I'm aiming a bit high asking for a whole panel, but yet it is an important topic, so why not state the ideal conditions? :)
The keyword for my own response and argument is going to be "cluster". I think it can and should be argued that there is a phenomenon of clustering that takes place with regard to moral behaviour choices.
So we can observe that most people believe that it is "wrong" to kill but also, sadly, that war is sometimes necessary and will almost certainly involve killing. In the same way, a police officer might have to kill a crazed gunman. Even though killing is wrong, there is some kind of justifying principle at work: the greater good (or the least harm). I note how this word "greater" operates. It presupposes that degrees of goodness and wrong can be identified in order to make the moral decision possible and indeed to make the moral decision itself.
Here is a question that made an alternative argument seem necessary for Christians (like myself, who don't think you can easily get to the Christian God by apologetic argumentation) to have another way into the debate: Can a theist imagine a functional intelligent society in which individuals, groups and the species as a whole could thrive without "God"? What would the minimum contents be of that God?
It's difficult to imagine, but this may depend on our definitions of God.
We accept that consciousness is a spectrum. Awareness varies according to the complexity of the life-form, and even self-awareness in humans can vary from non-existence to heightened (and back again, e.g. in a fit of rage). The moral code that says don't harm my own species is necessarily built into every species for it to advance.
Could we imagine a mass-mutation of a species away from that moral law? Yes! Unfortunately, it would also involve the self-destruction of the species.
As the species that is human, our accumulated knowledge continues to increase and our legal systems inevitably complexify in accordance with that knowledge, and the clustering process continues in new moral scenarios either hitherto unforeseen (e.g. medical ethics) or better understood (e.g. LGBQT).
So it is probably pretty clear that I find the presentation of John Cranman's (and WL Craig's) moral argument untenable in its current format. But I also don't think it shouldn't be abandoned altogether either, despite my previous doubts. It is simply that some tighter and also some looser definitions must be properly integrated. I also think it is useful to break the argumentation into two halves, as I have attempted below, to cater for intelligent and less intelligent species.
Here we go:
1. For a species to thrive at any level of intelligence requires ingrained knowledge of the behavioural norms necessary for the survival of any given individual's offspring, of the individual itself and of the individual's group.
2. Human beings are a species.
3. For humans to thrive requires their ingrained knowledge of the behavioural norms necessary for the survival of their offspring, themselves and of their group.
4. Such ingrained knowledge in humans can be aptly described as moral values and duties.
5. Although not strictly universal, moral values and duties can be observed to cluster around certain norms that transcend a given human social group.
6. That which transcends can be aptly described by an observer from within that species as equivalent to objective reality.
7. Moral values and duties are at least equivalent to an objective reality.
And so, at last, we arrive, as in the podcast episode, at the second premise first. Note that thus far an appeal to God has not seemed to be necessary for the flow of this argument.
H O W E V E R
8. Moral decisions can be complex, such as those involving conflicting moral values, and in such cases require more knowledge about likely outcomes in order to imagine and assess the "greater good" (or benefit) and the "greater harm".
9. Such an increased need of knowledge vastly exceeds the speed at which humans can evolve, and may even require total knowledge, letting us then assert such decisions require perfect knowledge on the part of the moral navigator in order to be made safely.
10. Only "God" has perfect knowledge (be he real or imagined).
11. Therefore God or an unrealised ideal knowledge is necessarily sought for complex moral choices to truly enhance the human race.
12. The Christian God is a perfectly legitimate solution to complex moral problems.
13. Non-theistic models of improving knowledge are also perfectly legitimate solutions to making better complex moral choices.
On the basis of the moral argument alone, we cannot state which offers the best explanation.