It is true that non-Christians, including atheists, can embrace natural law, as I discussed in an earlier post [here]. Not only that, but, as the radio hosts pointed out, St. Paul tells us:
For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified. When Gentiles who have not the law do by nature what the law requires, they are a law to themselves, even though they do not have the law. They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them . . . (Romans 2:13-15)
Our civilization has been shaped by Christianity for almost two thousand years. Christian beliefs, attitudes and moral convictions (commonly referred to as the Judeo-Christian worldview) are woven into all of our customs and institutions. It is true that we have never realized the Christian ideal; one could say that we haven’t even come close. Nevertheless, anyone raised in the West over the last two millenia has been formed, to a large degree, by that Christocentric worldview, whether they consciously embrace it or not. More than one commentator has remarked that even the “new atheists” such as Richard Dawkins and Christopher Hitchens have not jettisoned principles such as the dignity of the human person (not a universal value by any stretch) or a Judeo-Christian concept of justice — they employ these very ideas, in fact, as weapons against Christianity. This is confirmed by my personal experience: most self-proclaimed atheists with whom I am personally acquainted still adhere to a mostly Christian ethical code (even though they can’t give an authoritative reason why), with the usual exceptions involving sexual morality — which doesn’t do much to distinguish them from most professed Christians.
Without its source and foundation of Christian belief, however, the worldview itself will quickly wither . . .
That, I think, is the counter to the atheist’s challenge: it’s not that an atheist can’t be moral, but rather, without God, why should he?