3 So, then, if these things are so. we desire to learn this, first. from you— what is the cause, what the reason, that you offer them sacrifices; and then, what gain comes to the gods themselves from this, and remains to their advantage. For whatever is done should have a cause, and should not be disjoined from reason, so as to be lost among useless works, and tossed about among vain and idle uncertainties. Do the gods of heaven live on these sacrifices, and must materials be supplied to maintain the union of their parts?
And what man is there so ignorant or what a god is, certainly, as to think that they are maintained by any kind of nourishment, and that it is the food given to them which causes them to live and endure throughout their endless immortality? For whatever is upheld by causes and things external to itself, must be mortal and on the way to destruction, when anything on which it lives begins to be wanting. Again, it is impossible to suppose that any one believes this, because we see that of these things which are brought to their altars, nothing is added to and reaches the substance of the deities; for either incense is given, and is lost melting on the coals, or the life only of the victim is offered to the gods, and its blood is licked up by dogs; or if any flesh is placed upon the altars, it is set on fire in like manner, and is destroyed, and falls into ashes—unless perchance the god seizes upon the souls of the victims, or snuffs up eagerly the fumes and smoke which rise from the blazing altars, and feeds upon the odours which the burning flesh gives forth, still wet with blood, and damp with its former juices. But if a god, as is said, has no body, and cannot be touched at all, how is it possible that that which has no body should be nourished by things pertaining to the body—that what is mortal should support what is immortal, and assist and give vitality to that which it cannot touch? This reason for sacrifices is not valid, therefore, as it seems; nor can it be said by any one that sacrifices are kept up for this reason, that the deities are nourished by them, and supported by feeding on them.
Source: Against the Heathen (New Advent)