37 Since these things are so, and since there is so great difference between our opinions and yours, where are we, on the one hand, impious, or you pious, since the decision as to piety and impiety must be founded on the opinions of the two parties? For he who makes himself an image which he may worship for a god, or slaughters an innocent beast, and burns it on consecrated altars, must not be held to be devoted to religion. Opinion constitutes religion, and a right way of thinking about the gods, so that you do not think that they desire anything contrary to what becomes their exalted position, which is manifest. For since we see all the things which are offered to them consumed here under our eyes, what else can be said to reach them from us than opinions worthy of the gods, and most appropriate to their name? These are the surest gifts, these true sacrifices; for gruel, incense, and flesh feed the devouring flames, and agree very well with the parentalia of the dead.
Source: Against the Heathen (New Advent)