27 Finally, that we may always abide by the rule and definition by which it has been shown and determined that whatever is done by man must have its causes, we will hold it fast here also, so as to demand of you what is the cause, what the reason, that incense is put on the altars before the very images of the deities, and that, from its being burned, they are supposed to become friendly and gentle. What do they acquire from this being done, or what reaches their minds, so that we should be fight in judging that these things are well expended, and are not consumed uselessly and in vain?
For as you should show why you give incense to the gods, so, too, it follows that you should manifest that the gods have some reason for not rejecting it with disdain, nay more, for desiring it so fondly. We honour the gods with this, some one will perhaps say. But we are not inquiring what your feeling is, but the gods'; nor do we ask what is done by you, but how much they value what is done to purchase their favour. But yet, O piety, what or how great is this honour which is caused by the odour of a fire, and produced from the gum of a tree?
For, lest you should happen not to know what this incense is, or what is its origin, it is a gum flowing from the bark of trees, just as from the almond-tree, the cherry-tree, solidifying as it exudes in drops. Does this, then, honour and magnify the celestial dignities? Or, if their displeasure has been at any you excited, is it melted away before the smoke of incense, and lulled to sleep, their anger being moderated? Why, then, do you not burn indiscriminately the juice of any tree whatever, without making any distinction? For if the deities are honoured by this, and are not displeased that Panchaean gums are burned to them, what does it matter from what the smoke proceeds on your sacred altars, or from what kind of gum the clouds of fumigation arise?
Source: Against the Heathen (New Advent)