23 For as to that which we hear said by you, that some of the gods are good, that others, on the contrary, are bad, and rather inclined to indulge in wanton mischief, and that the usual rites are paid to the one party that they may show layout, but to the others that they may not do you harm—with what reason this is said, we confess that we cannot understand. For to say that the gods are most benevolent, and have gentle dispositions, is not only pious and religious, but also true; but that they are evil and sinister, should by no means be listened to, inasmuch as that divine power has been far removed and separated from the disposition which does harm. But whatever can occasion calamity, it must first be seen what it is, and then it should be removed very far from the name of deity.
Then, supposing that we should agree with you that the gods promote good fortune and calamity, not even in this case is there any reason why you should allure some of them to grant you prosperity, and, on the other hand, coax others with sacrifices and rewards not to do you harm. First, because the good gods cannot act badly, even if they have been worshipped with no honour.— for whatever is mild and placid by nature, is separated widely from the practice and devising of mischief; while the bad knows not to restrain his ferocity, although he should be enticed to do so with a thousand flocks and a thousand altars. For neither can bitterness change itself into sweetness, dryness into moisture, the heat of fire into cold, or what is contrary to anything take and change into its own nature that which is its opposite. So that, if you should stroke a viper with your hand, or caress a poisonous scorpion, the former will attack you with its fangs, the latter, drawing itself together, will fix its sting in you; and your caressing will be of no avail, since both creatures are excited to do mischief, not by the stings of rage, but by a certain peculiarity of their nature. It is thus of no avail to wish to deserve well of the sinister deities by means of sacrifices, since, whether you do this, or on the contrary do not, they follow their own nature, and by inborn laws and a kind of necessity are led to those things, to do which they were made. Moreover, in this way both kinds of gods cease to possess their own powers, and to retain their own characters. For if the good are worshipped that they may be favourable, and supplication is made in the same way to the others, on the contrary, that they may not be injurious, it follows that it should be understood that the propitious deities will show no favour if they receive no gifts, and become bad instead of good; while, on the contrary, the bad, if they receive offerings, will lay aside their mischievous disposition, and become thereafter good: and thus it is brought to this issue, that neither are these propitious, nor are those sinister: or, which is impossible, both are propitious, and both again sinister.
Source: Against the Heathen (New Advent)