As I say, then, atheism per se is not a direct threat to the very possibility of morality. Someone who denied the existence of God but accepted Aristotelian essentialism could have grounds for accepting at least part of the natural law. So too could someone who endorsed an atheistic form of Platonism (if there could be such a thing). But to opt for a completely anti-essentialist and anti-teleological view of the world -- one which holds that the natural order is entirely mechanistic and that there is nothing beyond that order -- is, the A-T philosopher would argue, to undermine the possibility of any sort of morality at all. For it entirely removes from the world essences and final causes, and thus the possibility of making sense of the good as an objective feature of reality. (See The Last Superstition for details.) And since modern atheism tends to define itself in terms of such a radically anti-teleological or mechanistic view of the world, it too is to that extent incompatible with any possible morality.I didn't even say it was incompatible with any morality. Just with objective morality. We could still construct a moral code from our reason but someone else could simply choose to ignore it. I am not sure if his definition of "any possible morality" would even exclude that.
But the point is if you don't just deny God but also deny anything supernatural then you have a huge moral problem. So many people treat the denial of anything beyond the scientifically verifiable as a kind of virtue. That they deal in real stuff and not that mumbo-jumbo. They have no idea that they have gotten rid of the concept of virtue. Just like the Nazi's and Communists had no idea where their atheism would lead. We think the modern atheism won't go there because we are somehow better than those movements were. Has the heart of man really changed?