10 But perhaps some one will say, We give to the gods sacrifices and other gifts, that, being made willing in a measure to grant our prayers, they may give us prosperity and avert from us evil, cause us to live always happily, drive away grief truly, and any evils which threaten us from accidental circumstances. This point demands great care; nor is it usual either to hear or to believe what is so easily said. For the whole company of the learned will straightway swoop upon us, who, asserting and proving that whatever happens, happens according to the decrees of fate, snatch out of our hands that opinion, and assert that we are putting our trust in vain beliefs.
Whatever, they will say, has been done in the world, is being done, and shall be done, has been settled and fixed in time past, and has causes which cannot be moved, by means of which events have been linked together, and form an unassailable chain of unalterable necessity between the past and the future. If it has been determined and fixed what evil or good should befall each person, it is already certain; but if this is certain and fixed, there is no room for all the help given by the gods, their hatred, and favours.
For they are just as unable to do for you that which cannot be done, as to prevent that from being done which must happen, except that they will be able, if they choose, to depreciate somewhat powerfully that belief which you entertain, so that they say that even the gods themselves are worshipped by you in vain, and that the supplications with which you address them are superfluous. For as they are unable to turn aside the course of events, and change what has been appointed by fate, what reason, what cause, is there to wish to weary and deafen the ears of those in whose help you cannot trust at your utmost need?
Source: Against the Heathen (New Advent)