The clearest way to show this is to give the various arguments that have actually been used to prove the claim there can be no morality or moral action without God.
1.) Morality is rational, but the existence of God is a necessary practical postulate for rational morality. (Kant)
2.) Human nature is wounded, by which we mean incapable of acting morally without some exterior supernatural aid. (St. Paul, interpretation 1)
3.) Human nature is called to live with a being whose existence exceeds what it can discovered by its own rational power, therefore human nature cannot achieve what it is called to unless this being reveals itself to him. But moral action consists in achieving that which one is called to do, in the sense of perfecting ones nature. (St. Paul, interpretation 2)
4.) Morality requires keeping the weak-minded in line. But one cannot keep the weak minded in line without scaring them with the idea of divine things. (A common opinion of the Roman consuls who killed the Christians)
5.) Justice, the form of the virtues, is nothing but gazing upon the good itself, that is, the philosophical gazing on the divine absolute. (Plato)
6.) The action of anything is its way of trying to become as much like God as possible, and morality is human action (Aristotle)
7.) Of itself, nature is meaningless. But morality of itself is not meaningless. Therefore morality is from a supernatural source, and whatever is from a supernatural source is from God (William Lane Craig)
8.) Of itself, nature is meaningful. But the meaningfulness of artifacts must be reduced to some intelligent being, therefore the meaningfulness of nature must be reduced to an intelligent being. But morality is meaningful action.
9.) Morality is partaking in justice itself, good itself, etc. But God is subsistent justice itself, goodness itself, etc. (Neo-Platonism, sympathetic with Thomism)
10.) Here are the most celebrated atheist regimes. Almost all of them are moral monsters.
11.) Moral action is doing what the herd does. But the herd of Western civilization is founded on belief in God and has not yet given itself a new foundation (suggested by Nietzsche, but not him exactly)
12.) Moral action must satisfy natural desires. But man has a natural desire for God.
13.) Moral action requires knowing what you were made for and what your purpose is. But only your maker and creator knows what your purpose is, and you were specially made and created by God.
So that’s a baker’s dozen, in no particular order. All of them have enjoyed widespread popularity for one reason or another, either by being preacher’s enthymemes or by being the thoughts of a major school or popular thinker. Yet only one of them is a divine command theory (7) and it is not even the paradigm case of the divine command theory (which we find, as Brandon has been saying for years and deserves to be listened to, in William Warburton) There is, in fact, a “default setting” in the Western mind to tie morality to the divine, but divine command theory is only one attempt to articulate this default setting. And so the desire of the Western man’s psyche (and, as Wilkins suggests, this might be a more universal desire) to tie his morality to the divine seems less like a theory and more like a universal sense that articulates itself imperfectly in many different theories.
Morality consists in doing ones conscience, conscience is the voice of God (Newman and Frankl)