5 But let it be assumed that there are these gods, as you wish and believe, and are persuaded; let them be called also by those names by which the common people suppose that those meaner gods are known. Whence, however, have you learned who make up the list of gods under these names? have any ever become familiar and known to others with whose names you were not acquainted? For it cannot be easily known whether their numerous body is settled and fixed in number; or whether their multitude cannot be summed up and limited by the numbers of any computation.
For let us suppose that you do reverence to a thousand, or rather five thousand gods; but in the universe it may perhaps be that there are a hundred thousand; there may be even more than this—nay, as we said a little before, it may not be possible to compute the number of the gods, or limit them by a definite number. Either, then, you are yourselves impious who serve a few gods, but disregard the duties which you owe to the rest; or if you claim that your ignorance of the rest should be pardoned, you will procure for us also a similar pardon, if in just the same way we refuse to worship those of whose existence we are wholly ignorant.
Source: Against the Heathen (New Advent)