In my recent article, I pointed out that, using “know” as “justified true belief,” an atheist does not know many divine commands that constitute his moral obligations. In that sense, then, the answer to the question, “does an atheist need God in order to know his moral duties?” is “yes,” at least in order to have a fully robust account of moral obligations.
However, it occurred to me tonight that we can generate a problem for the “ought implies can” principle. Ought implies can states that if one ought to do something, then he can do something; a corollary of this principle is that if he cannot do something, then it is not the case that he ought to do it. In short, if you cannot fulfill a moral obligation, then the supposed moral obligation is not binding upon you. So what’s the problem here?
If atheists do not know certain of their moral duties, how can they be held responsible for not fulfilling them? It seems obvious that knowing a moral law is a necessary condition for fulfilling the entailed moral obligation. If the necessary condition is not present, then they cannot fulfill that moral obligation. If they cannot fulfill it, then, by ought-implies-can, they are not obligated to do it. This would place atheists off the hook for just anything they did not believe.
This obviously seems silly: who would think that by denying certain moral truths and entailed obligations we have thereby divested ourselves of those particular obligations? So, apparently ought-implies-can must go, right? Not so fast. Instead, consider this response: ignorance of the law is no excuse. Now this seems to contradict what we have written above.
I submit there are (at least) two types of ignorance. First, there is the type of ignorance that is uninformed; the subject simply lacks the propositional or intuitional knowledge that constitutes his moral obligation. Second, there is what the Bible calls “willful ignorance.” 2 Peter 3:5 states, “For this they are willingly ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water.” Willful ignorance, biblically and philosophically, is a lack of knowledge done via culpability. This explains our strong intuition that these obligations are incumbent upon everyone, even if he does not believe it.
As n. 2 explains below, specific moral duties are understood as applications of objective moral values. Since most everyone apprehends these values, the applications of specific moral values into duties are therefore incumbent upon everyone. In short, it is not a necessary condition that the subject believes he is obligated to do such-and-such. Rather, the necessary condition is whether or not he knows, or has an obligation to know, such moral obligations. And that, I think, is just obvious. If one perceives moral values, and there is a moral law given by God, then it follows that one has moral obligations. Surely it is one’s duty to know these commands by means of belief once they are presented. But it is precisely this duty of belief that is shrugged aside by the non-Christian. This makes him culpable, and it preserves ought-implies-can in the relevant conversation.
 For instance, we would not want to say that Jim is morally praiseworthy for paying his bills on time if he has absolutely no idea that paying his bills on time is a moral obligation for Jim. While this is silly, the idea is that someone truly fulfilling the moral law cannot be a happy accident (where S is X’s obligation, X performs S, but has no thoughts nor knowledge of S at any time during X’s performing S).
 Here, we must be very careful to distinguish moral values and duties. Moral duties are derived (understood) as applications of moral values. In this way, a subject need not be exposed, propositionally, to every instantiation of moral obligation explicitly. It is his responsibility to derive those applications and to do them.