Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 21

Chapter 21.—That the Roman Dominion Was Granted by Him from Whom is All Power, and by Whose Providence All Things are Ruled.

These things being so, we do not attribute p. 103 the power of giving kingdoms and empires to any save to the true God, who gives happiness in the kingdom of heaven to the pious alone, but gives kingly power on earth both to the pious and the impious, as it may please Him, whose good pleasure is always just.  For though we have said something about the principles which guide His administration, in so far as it has seemed good to Him to explain it, nevertheless it is too much for us, and far surpasses our strength, to discuss the hidden things of men’s hearts, and by a clear examination to determine the merits of various kingdoms.  He, therefore, who is the one true God, who never leaves the human race without just judgment and help, gave a kingdom to the Romans when He would, and as great as He would, as He did also to the Assyrians, and even the Persians, by whom, as their own books testify, only two gods are worshipped, the one good and the other evil,—to say nothing concerning the Hebrew people, of whom I have already spoken as much as seemed necessary, who, as long as they were a kingdom, worshipped none save the true God.  The same, therefore, who gave to the Persians harvests, though they did not worship the goddess Segetia, who gave the other blessings of the earth, though they did not worship the many gods which the Romans supposed to preside, each one over some particular thing, or even many of them over each several thing,—He, I say, gave the Persians dominion, though they worshipped none of those gods to whom the Romans believed themselves indebted for the empire.  And the same is true in respect of men as well as nations.  He who gave power to Marius gave it also to Caius Cæsar; He who gave it to Augustus gave it also to Nero; He also who gave it to the most benignant emperors, the Vespasians, father and son, gave it also to the cruel Domitian; and, finally, to avoid the necessity of going over them all, He who gave it to the Christian Constantine gave it also to the apostate Julian, whose gifted mind was deceived by a sacrilegious and detestable curiosity, stimulated by the love of power.  And it was because he was addicted through curiosity to vain oracles, that, confident of victory, he burned the ships which were laden with the provisions necessary for his army, and therefore, engaging with hot zeal in rashly audacious enterprises, he was soon slain, as the just consequence of his recklessness, and left his army unprovisioned in an enemy’s country, and in such a predicament that it never could have escaped, save by altering the boundaries of the Roman empire, in violation of that omen of the god Terminus of which I spoke in the preceding book; for the god Terminus yielded to necessity, though he had not yielded to Jupiter.  Manifestly these things are ruled and governed by the one God according as He pleases; and if His motives are hid, are they therefore unjust?