Firstly, whether an idea is true or not does not depend on the fact that it serves my interest (or not). In actual fact, it is totally independent. Obviously, it is in our interest to believe that God exists and to be moral: this is the reason why this idea is readily imagined and accepted. Nietzsche remarks with reason that we cannot conclude that this idea is true: it is not because a theory serves our interest that it is true. However, it would be just as wrong to infer the opposite, namely that a theory serving our interest is thereby wrong. It is not because it is good for us that a God exists that we should conclude that He does not exist. It is not because it is in our interest to be moral that we should not make it our duty.
Consequently, whether a given idea does or does not conflict with our self-interest, we cannot conclude anything as to its truth or to its foundation. Even so, it teaches us something about the popularity of this idea, viz. the importance we give it: a theory serving our interest is broadly shared. It can also reveal how this idea has come to mind: Have I learnt it from society or from my weakness? Here again, nothing is mentioned about its truth or foundation.
Furthermore, the Nietzschean doctrine is based on an aristocratic axiological postulate, which we can call into question: the idea that what has a contemptible origin is, in itself, contemptible. However, we commonly observe the opposite. Certain things have much more value than what is at their origin. For example, a tumultuous river arises from a tiny source, a great man like Napoleon comes from a modest Corsican family, Nietzsche himself was born in a pious family and his father was a pastor. Let us generalise this idea: it is not because the morality and the idea of God have arisen from a despicable self-interest that they are actually despicable.
Lastly, the Nietzschean position depends on an axiological judgment, which is dogmatic, for it is not founded: weakness is worthless, whereas strength has a great value. It may be true, but it may not be. On the other hand, we can imagine the axiological position in which fragility, weakness, and daintiness have a value (for example, a fawn, a flower alone in a field, a child, etc.), and the fact is that it is often this kind of thing which pleases us.
To conclude, in his genealogical investigation, Nietzsche shows us how the ideas of morality and religion came to our knowledge and why they interest us. Their negative value, such as their falseness, or their foundation, are inferred from their origin (but such an inference is impossible), and from a dogmatic axiological judgment, which has no basis. Due to the multiplicity of different value judgments that we can oppose , we can raise the problem of values.
This vain attempt to define the foundation of morality from its origin may be observed in other doctrines. A prime example is what I refer to as “moral sociologism”.
d/ Extension to moral sociologism
By moral sociologism, I am referring to the doctrine which attempts to justify or refute, viz. to identify such and such moral rules by reducing them to their social causes or conditions. A first given moral rule is approved because the state of society makes its adoption necessary, in other words, because a deep social cause is at the origin of its institution. On the other hand, a second rule is criticised, because it is noted that society has changed, and as the social cause which gave rise to this rule has disappeared, the rule in question must be abandoned.
Therefore, we can see that this doctrine consists in deducing the foundation (or lack of foundation) of a morality from its origin – in this case, its social origin - as the cause (or condition) of a phenomenon is nothing other than its origin.
It is likely that a vast number of sociologists do not feel concerned by the doctrine of “moral sociologism”. However, it is noted that this doctrine has been formulated and conceptualised by one of the fathers of sociology, Durkheim, and it can be identified in a great number of books about Sociology. I believe that it is legitimate to call “moral sociologism” a doctrine, which attempts to determine the foundation of morality from the society (considered as standard) in which this morality has developed.