There is a very strange error which many atheists make when debating theists: they think that the word morality means no more than, “how you make decisions”. They will then propose some means by which they make decisions and say that this shows that atheists can be moral too. These rules never mandate nor forbid anything, of course, and always seem suspiciously like what somebody raised with a real moral code would find comfortable, supposing that they’re reasonably well-to-do and live in a peaceful place with little crime.
I recently saw an example where somebody proposed the golden rule, which he claimed required no God. Of course there is absolutely no reason given why one should obey it, but for the moment, let’s ignore that. Suppose that the following were true:
If I were rich and owned a bank, I would really like it if people tried to rob my bank at gunpoint so I could have the fun of patrolling the branches to heroically stop the robbery should I arrive at the right time.
The conclusion, then, would be that a man who felt this way should go and rob banks at gunpoint. No God required.
For the moment, let’s leave voluntarism out of account since anyone who believes in voluntarism has explicitly rejected reason anyway and so can’t be reasoned with (voluntarism is the idea that morality flows from God’s will rather than his intellect, so God could command rape and murder and forbid kindness and mercy). The only way to get an actual morality which both has both positive and negative commands and actually works is for it to be grounded in the nature of things to which the moral rules apply. I’ll give a fuller description of this later, but the short version is that all sin is a diminishment of being. God is love, which means that God is generosity, and in his generosity he has given it to us to be his generosity to the world. He could give my children food directly, but instead has given it to me to be his gift of food to my children. He could have created them directly, but gave it to my wife and I to be his act of creating them. To those of us who pass hungry beggars on the street, he gives it to us to be his gift of food to them. To those of us with tongues he gives it to us to be his speaking of the truth to those with ears to hear it. And so it goes for all moral rules: it is our nature to be God’s act of generous creation to the world, in ways big and small. To tell someone the truth is to create in them knowledge. And so it goes with all things we do that are good.
To sin is to refuse to do this work of creation we were given to do. Being is good, so to refuse to do this work is to diminish being, and is therefore evil because there is less good. (Evil is a negative, not a positive, thing, and has no existence on its own. Evil exists only in the manner of a shadow, which “exists” only where the light does not hit.)
All actually grounded moralities must have this in common as their ground. It is of course possible to take a morality on faith, without understanding its grounding, but it must of necessity come back to some ultimate source for our existence. Atheists will never succeed in grounding a real morality because they do not believe in a reality capable of grounding a morality. Blind matter mechanically acting so arbitrary rules has no further being than merely existing. We might think particular configurations of it interesting, or like them, but this is merely to be entertained by illusions. To have a real morality, you need a real reality.