Kant, as I say, invented a new moral argument for the existence of God, and that in varying forms was extremely popular during the nineteenth century, It has all sorts of forms. One form is to say that there would be no right or wrong unless God existed. I am not for the moment concerned with whether there is a difference between right and wrong, or whether there is not: that is another question. The point I am concerned with is that, if you are quite sure there is a difference between right and wrong, you are then in this situation: Is that difference due to God's fiat or is it not? If it is due to God's fiat, then for God himself there is no difference between right and wrong, and it is no longer a significant statement to say that God is good. If you are going to say, as theologians do, that God is good, you must then say that right and wrong have some meaning which is independent of God's fiat, because God's fiats are good and not bad independently of the mere fact that he made them. If you are going to say that, you will then have to say that it is not only through God that right and wrong came into being, but that they are in their essence logically anterior to God. You could, of course, if you liked, say that there was a superior deity who gave orders to the God who made this world, or could take up the line that some of the gnostics took up (a line which I often thought was a very plausible one), that as a matter of fact this world that we know was made by the devil at a moment when God was not looking. There is a good deal to be said for that, and I am not concerned to refute it.
Secondly, A. J. Ayer added (in his essay "The Claims of Theology" ...
Whichever way it is taken, this proposition contains two serious errors, apart from the fallacy of thinking that the absence of grounds for morality entails the absence of motives. The first error is to suppose morality needs an ulterior justification. The second error is to suppose that a God could supply it. The fallacy which is involved in thinking morals could be founded on divine authority has been exposed by many philosophers, but perhaps most clearly and succinctly by Russell.
"Theologians have always taught that God's decrees are good, and that this is not a mere tautology: it follows that goodness is logically independent of God's decrees."
The point is that moral standards can never be justified merely by an appeal to authority, whether the authority is taken to be human or divine. There has to be the additional premise that the person whose dictates we are to follow is good, or what he commands is right, and this cannot be the mere tautology that he is what he is, or that he commands what he commands. This does not mean that we cannot look for guidance in conduct to those whom we judge to be better or wiser or more experienced than ourselves. To a greater or lesser extent, we can and do take our morals on trust but in so doing we are making a moral decision. We are at least implicitly judging that the rules which we have been brought up to respect or the verdicts of our mentor are morally right: and again this is not the mere tautology that these rules and verdicts just are what they are.
But if a moral code cannot be founded on authority, neither can it be founded on metaphysics or on science or on empirical matters of fact Scientific and factual considerations are indeed relevant to morals because of the bearing which they have upon the application of our moral principles. We have to know what the situation is in which we are placed and what the consequences of different actions are likely to be. If, for example, we think it right to try to maximize human happiness, a scientific approach to the practical problems may instruct us how best to set about it. The adoption of such a principle is, however, something which is not dictated to us by the facts. It is a decision for which it may be that we are not able to give any further reason, just as we may not be able to give any further reason for the value that we attach to justice or to liberty. In the end, it is a matter of finding principles which one is prepared to stand by and when they conflict, as for most of us they sometimes will, of giving more weight to one or another according to the circumstances of the particular case.