Celsus, then, as if not observing that he was saying anything inconsistent with the words he had just used, “if all were to do the same as you,” adds: “You surely do not say that if the Romans were, in compliance with your wish, to neglect their customary duties to gods and men, and were to worship the Most High, or whatever you please to call him, that he will come down and fight for them, so that they shall need no other help than his. For this same God, as yourselves say, promised of old this and much more to those who served him, and see in what way he has helped them and you!
They, in place of being masters of the whole world, are left with not so much as a patch of ground or a home; and as for you, if any of you transgresses even in secret, he is sought out and punished with death.” As the question started is, “What would happen if the Romans were persuaded to adopt the principles of the Christians, to despise the duties paid to the recognised gods and to men, and to worship the Most High?” this is my answer to the question. We say that “if two” of us “shall agree on earth as touching anything that they shall ask, it shall be done for them of the Father” of the just, “which is in heaven;” for God rejoices in the agreement of rational beings, and turns away from discord.
And what are we to expect, if not only a very few agree, as at present, but the whole of the empire of Rome? For they will pray to the Word, who of old said to the Hebrews, when they were pursued by the Egyptians, “The Lord shall fight for you, and you shall hold your peace;” and if they all unite in prayer with one accord, they will be able to put to flight far more enemies than those who were discomfited by the prayer of Moses when he cried to the Lord, and of those who prayed with him.
Now, if what God promised to those who keep His law has not come to pass, the reason of its nonfulfilment is not to be ascribed to the unfaithfulness of God. But He had made the fulfilment of His promises to depend on certain conditions—namely, that they should observe and live according to His law; and if the Jews have not a plot of ground nor a habitation left to them, although they had received these conditional promises, the entire blame is to be laid upon their crimes, and especially upon their guilt in the treatment of Jesus.
Source: Against Celsus (New Advent)