17 Let no man tell you fables, then. “Pontius wrought a miracle; and Donatus prayed, and God answered him from heaven.” In the first place, either they are deceived, or they deceive. In the last place, grant that he removes mountains: “And have not charity,” says the apostle, “I am nothing.” Let us see whether he has charity. I would believe that he had, if he had not divided unity. For against those whom I may call marvel-workers, my God has put me on my guard, saying, “In the last times there shall arise false prophets, doing signs and wonders, to lead into error, if it were possible, even the elect: Lo, I have foretold it to you.” Therefore the Bridegroom has cautioned us, that we ought not to be deceived even by miracles.
Sometimes, indeed, a deserter frightens a plain countryman; but whether he is of the camp, and whether he is the better of that character with which he is marked, is what he who would not be frightened or seduced attends to. Let us then, my brethren, hold unity: without unity, even he who works miracles is nothing. The people Israel was in unity, and yet wrought no miracles: Pharaoh's magicians were out of unity, and yet they wrought the like works as Moses. The people Israel, as I have said, wrought no miracles.
Who were saved with God— they who did, or they who did not, work miracles? The Apostle Peter raised a dead person: Simon Magus did many things: there were there certain Christians who were not able to do either what Peter did or what Simon did; and wherein did they rejoice? In this, that their names were written in heaven. For this is what our Lord Jesus Christ said to the disciples on their return, because of the faith of the Gentiles. The disciples, in truth, themselves said, boasting, “Behold, Lord, in Your name even the devils are subject to us.”
Rightly indeed they confessed, they brought the honor to the name of Christ; and yet what does He say to them? “Do not ye glory in this, that the devils are subject to you; but rejoice that your names are written in heaven.” Peter cast out devils. Some old widow, some lay person or other, having charity, and holding the integrity of faith, forsooth does not do this. Peter is the eye in the body, that man is the finger, yet is he in the same body in which Peter is; and if the finger has less power than the eye, yet it is not cut off from the body. Better is it to be a finger and to be in the body, than to be an eye and to be plucked out of the body.
Source: Tractates on the Gospel of John (New Advent)