First of all, what can prove that we are self-interested in an idea as to its truth or falsity? Nothing, it seems. Of course, we have a self-interest in believing that God exists, and that we should be moral: that is why this idea is easily imagined and readily accepted. Nietzsche rightly points out that it would be wrong to infer that it is true: just because an idea serves our interests does not mean that it is true. However, it would be just as wrong to infer that an idea is false because it serves our interests. It is not because it would be good for us if a God existed that we should infer that he does not exist. Just because it is in our interest to be moral does not mean we should not be.
Establishing the value of an idea does not tell us anything about its value or truth, i.e. its foundation, but only about its success with us, and the importance we attach to it.
On the other hand, the origin of an idea does not tell us anything about its truth or falsity; it tells us how it came to our knowledge. Whether we learnt it from our weakness or from society does not tell us anything about its truth or value.
Moreover, Nietzsche's doctrine involves an aristocratic axiological postulate that can be called into question, according to which what comes from a despicable origin can only be despicable in itself. Yet it is commonplace to see that what comes from a despicable origin can infinitely exceed in value what is at its origin: thus the tumultuous river comes from the tiny source, a great man like Napoleon comes from a modest family in Corsica, Nietzsche himself comes from a deeply pious family whose father was a pastor, and so on. So just because morality and the idea of God are born of a petty self-interest does not mean that they are in turn petty.
Finally, Nietzsche's condemnation is based on an axiological judgement that seems unfounded to me, and therefore has the value of dogma, according to which weakness has a negative value, whereas power has a great value. This judgement may be true or false, but in any case it is unfounded. On the other hand, we can imagine a position that affirms that it is gentleness, fragility and weakness that have a value (that of a fawn, a flower isolated in the middle of a field, a child, etc.), and it is often in fact this that appeals to us.
To sum up, Nietzsche's genealogical study shows us how the ideas of morality and religion come to our knowledge, and why we take such an interest in them; their negative value in itself (or their falsity) is asserted on the basis of an impossible inference (from the origin to the foundation of an idea) and of a dogmatic axiological judgement against which others can be set, the multiplicity of which leads consciousness precisely to pose the problem of values.
I have tried to show that this attempt - doomed to failure - to deduce the basis of morality from its origin does not seem to characterise Nietzsche's work alone. In fact, a large number of doctrines seem to me to participate in this approach, first and foremost what we might call 'moral sociologism'.
d/ Extension to moral sociologism
Moral sociologism could be defined as the doctrine which attempts to justify or blame, i.e. to found, such and such a rule of morality, by reducing it to its social cause - or conditions. A particular moral rule will be praised by showing that it is required by the very state of society, i.e. that a profound social cause is at the origin of the establishment of this rule, and conversely (an approach more frequently followed) another moral rule will be criticised by arguing that society has evolved, and that the social cause which gave rise to this rule no longer exists, so that this rule must be abandoned.
We can see that this doctrine does indeed consist in deducing the foundation (or absence of foundation) of a morality from its origin - its social origin - since the cause (or condition) of a phenomenon is nothing other than its origin.
It is probable that a large number of sociologists do not recognise themselves in any way in this doctrine of 'moral sociologism'. However, it is notable that this doctrine was formulated and conceptualised by one of the fathers of sociology, Durkheim, and that it is frequently found in a large number of sociological works. Finally, I thought it legitimate to call 'moral sociologism' a doctrine that attempts to determine the basis of morality according to the canon constituted by the society in which that morality is exercised.