72 But, says Faustus, it cannot be admitted that the true God, who is also good, ever gave such a command. I answer, such a command can be rightly given by no other than the true and good God, who alone knows the suitable command in every case, and who alone is incapable of inflicting unmerited suffering on any one. This ignorant and spurious goodness of the human heart may as well deny what Christ says, and object to the wicked being made to suffer by the good God, when He shall say to the angels, "Gather first the tares into bundles to burn them."
The servants, however, were stopped when they wished to do this prematurely: "Lest by chance, when you would gather the tares, you root up the wheat also with them." Thus the true and good God alone knows when, to whom, and by whom to order anything, or to permit anything. In the same way, this human goodness, or folly rather, might object to the Lord's permitting the devils to enter the swine, which they asked to be allowed to do with a mischievous intent, especially as the Manichæans believe that not only pigs, but the vilest insects, have human souls.
But setting aside these absurd notions, this is undeniable, that our Lord Jesus Christ, the only son of God, and therefore the true and good God, permitted the destruction of swine belonging to strangers, implying loss of life and of a great amount of property, at the request of devils. No one can be so insane as to suppose that Christ could not have driven the devils out of the men without gratifying their malice by the destruction of the swine. If, then, the Creator and Governor of all natures, in His superintendence, which, though mysterious, is ever just, indulged the violent and unjust inclination of those lost spirits already doomed to eternal fire, why should not the Egyptians, who were unrighteous oppressors, be spoiled by the Hebrews, a free people, who would claim payment for their enforced and painful toil, especially as the earthly possessions which they thus lost were used by the Egyptians in their impious rites, to the dishonor of the Creator?
Still, if Moses had originated this order, or if the people had done it spontaneously, undoubtedly it would have been sinful; and perhaps the people did sin, not in doing what God commanded or permitted, but in some desire of their own for what they took. The permission given to this action by divine authority was in accordance with the just and good counsel of Him who uses punishments both to restrain the wicked and to educate His own people; who knows also how to give more advanced precepts to those able to bear them, while He begins on a lower scale in the treatment of the feeble. As for Moses, he can be blamed neither for coveting the property, nor for disputing, in any instance, the divine authority.
Source: Reply to Faustus the Manichaean (New Advent)