Objection 1: It seems that demons cannot lead people astray through real miracles. For the activity of demons will be especially evident in the works of the Antichrist. But as the Apostle says (2 Thess 2:9), his “coming is according to the working of Satan, with all power and signs and lying wonders.” Much more, then, do demons perform lying wonders at other times.
Objection 2: Furthermore, true miracles are effected through a physical change. But demons are incapable of changing the nature of a body; for Augustine says (De Civ. Dei xviii, 18): “I cannot believe that the human body can, by the art or power of a demon, take on the limbs of an animal.” Therefore, demons cannot perform real miracles.
Objection 3: Furthermore, an argument is useless if it can prove both ways. So if real miracles can be performed by demons to convince someone of something false, they will be useless for confirming the teaching of the faith. This is inappropriate; for it is written (Mk 16:20): “The Lord worked with them and confirmed the word by the signs that followed.”
On the contrary, Augustine says (Question [83]; [*Lib. xxi, Sent. sent 4, among the spurious works of St. Augustine]): “Often, by magical art, wonders are performed similar to those performed by the servants of God.”
I answer that, As is clear from what has been said above (Question [110], Article [4]), if we take a miracle in the strict sense, demons cannot perform miracles, nor can any creature, but God alone: since a miracle in the strict sense is something done outside the order of the entire created nature, under which order every power of a creature is contained. But sometimes miracle can be understood in a broad sense, for anything that surpasses human power and experience. And so demons can perform miracles, that is, things that arouse human astonishment because they surpass his power and lie outside his knowledge. For even a man who does what surpasses the power and knowledge of another causes that man to marvel at what he has done, so that he seems in some way to have performed a miracle. It should be noted, however, that although these works of demons, which appear wonderful to us, are not real miracles, they are nevertheless sometimes something real. Thus, the magicians of Pharaoh, by the power of demons, produced real snakes and frogs. And “when fire fell from heaven and at once consumed Job’s servants and sheep; when the storm threw down his house and with it his children—these were works of Satan, not mere illusions,” as Augustine says (De Civ. Dei xx, 19).
Reply to Objection 1: As Augustine says in the same place, the works of the Antichrist can be called lying wonders, “either because he will deceive the senses of men by illusions, so that he does not really do what he seems to do; or because, if he performs real miraculous signs, these will lead into falsehood those who believe in him.”
Reply to Objection 2: As we said above (Question [110], Article [2]), bodily matter does not obey either good or evil angels at their will, so that demons could by their power change matter from one form to another; but they can use certain seeds that exist in the elements of the world to produce these effects, as Augustine says (De Trin. ii, 8,9). Therefore, it must be admitted that all transformations of bodily things that can be produced by certain natural forces, to which we must assign the aforementioned seeds, can equally be produced by the activity of demons using these seeds; such as the transformation of certain things into snakes or frogs, which can arise through putrefaction. On the other hand, those transformations that cannot be produced by the power of nature cannot in reality be effected by the activity of demons; for example, that the human body be changed into the body of an animal or that the body of a dead man return to life. And if at times something of this sort seems to be effected by the activity of demons, it is not real, but a mere appearance of reality.
Now this can happen in two ways. First, from within; in this way, a demon can act upon a man’s imagination and even upon his bodily senses, so that something appears different from what it is, as explained above (Question [111], Article [3], [4]). Indeed, it is said that this can sometimes be done by the power of certain bodies. Second, from without: for just as he can form a body of any shape and appearance out of air and assume it to appear visibly, so he can in the same way clothe any bodily thing with any bodily form to appear in it. This is what Augustine says (De Civ. Dei xvii, 18): “The imagination of man, whether thinking or dreaming, takes on the forms of an innumerable number of things, and appears to the senses of other men, as if embodied in the likeness of some animal.” This is not to be understood as if the imagination itself or the images formed in it are identical with what appears embodied to the senses of another man; but that the demon, forming an image in the imagination of one man, can present the same image to the senses of another man.
Reply to Objection 3: As Augustine says (Questions 83, q. 79): “When magicians do what holy men do, they do it for a different purpose and by a different right. The former do it for their own glory; the latter for the honor of God: the former by certain private pacts; the latter by the manifest assistance and command of God, to whom every creature is subject.”
Source: Summa Theologiae, First Part, Question 114, Article 4