38 How, then, can you give to religion its whole power, when you fill into error about the gods themselves? Or summon us to their solemn worship, while you give us no definite information how to conceive of the deities themselves? For, to take no notice of the other authors, either the first makes away with and destroys six divine Muses, if they are certainly nine; or the last adds six who have no existence to the three who alone really are; so that it cannot be known or understood what should be added, what taken away; and in the performance of religious rites we are in danger of either worshipping that which does not exist, or passing that by which, it may be, does exist.
Piso believes that the Novensiles are nine gods, set up among the Sabines at Trebia. Granius thinks that they are the Muses, agreeing with Aelius; Varro teaches that they are nine, because, in doing anything, that number is always reputed most powerful and greatest; Cornificius, that they watch over the renewing of things, because, by their care, all things are afresh renewed in strength, and endure; Manilius, that they are the nine gods to whom alone Jupiter gave power to wield his thunder. Cincius declares them to be deities brought from abroad, named from their very newness, because the Romans were in the habit of sometimes individually introducing into their families the rites of conquered cities, while some they publicly consecrated; and lest, from their great number, or in ignorance, any god should be passed by, all alike were briefly and compendiously invoked under one name— Novensiles.
Source: Against the Heathen (New Advent)