The Moral Arguments for the existence of God follow from Immanuel Kant in his Critique of Pure Reason. In short the argument says that there would be no right or wrong without the existence of God.
Russell answers this argument by asking the following question to a theist, assuming he is quite sure that there is a difference between right and wrong: is the difference between right and wrong imposed by God or does it exist independently of Him. Russell argues as follows:
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 fiat 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.
Russell's argument can be answered on the following ground. An object A can have a property a that does not exist independently of A. In such a case A is uniquely but not completely determined by property a. Suppose now we have a different object B with a property b that is not unique to B. It is clear that B does not possess a. However, b can be said to be similar to a by an observer to the degree that the observer can discern properties a and b of the objects A and B.
First of all, we should clear up the difference between right and wrong for humans and in the absolute sense of the word. Feeding raw fish to a human infant will kill him, not feeding raw fish to an infant shark will kill it. If there are sets of ``rights'' and ``wrongs'' regarding feeding of infant humans and sharks, they will be diametrically opposite to each other. Some creatures thrive on sulphur dioxide which is lethal for humans. This simple example shows that we cannot extrapolate things that are right or wrong for humans to be the absolute rights or absolute wrongs. God is for all creatures, therefore the sense of right or wrong He commands must be absolute. Since nothing else is absolute, His qualities are unique to Him, and this defines the concept of right for God. So if we say that God is good, then this quality is unique to Him and does not exist independently of Him. Therefore, there is no point where a fiat is exercised.
Moreover, since we define right and wrong as it pertains to us, and cannot perceive things in their entirety, our definitions of right and wrong are necessarily similitudes to the absolutes. The absolute concepts are far removed from human ideas.