So the demons begged Jesus, “If You drive us out, send us into the herd of pigs.”
Matthew 8:31. Permit us to depart into the herd of swine Some conjecture that they wished to attack the swine, because they are filled with enmity to all God’s creatures. I do admit it to be true, that they are entirely bent on confounding and overthrowing the whole order of nature which God has appointed.
We have here the story of Christ's casting the devils out of two men that were possessed. The scope of this chapter is to show the divine power of Christ, by the instances of his dominion over bodily diseases, which to us are irresistible; over winds and waves, which to us are yet more uncontrollable; and lastly, over devils, which to us are most formidable of all.
Commenting on Matthew 8:28-34
And he said unto them, go, &c. He gave them leave, as God did to Satan, in the case of Job; for without divine permission, these evil spirits cannot do anything to the bodies, souls, or estates of men: they could not enter into the swine without leave, and much less do things of greater moment and consequence; and therefore are not to be feared...