Objection 1. It would seem that the demons cannot lead men astray by means of real miracles. For the activity of the demons will show itself especially in the works of Antichrist. But as the Apostle says (2 Thessalonians 2:9), his "coming is according to the working of Satan, in all power, and signs, and lying wonders." Much more therefore at other times do the demons perform lying wonders.
Objection 2. Further, true miracles are wrought by some corporeal change. But demons are unable to change the nature of a body; for Augustine says (De Civ. Dei xviii, 18): "I cannot believe that the human body can receive the limbs of a beast by means of a demon's art or power." Therefore the demons cannot work real miracles.
Objection 3. Further, an argument is useless which may prove both ways. If therefore real miracles can be wrought by demons, to persuade one of what is false, they will be useless to confirm the teaching of the faith. This is unfitting; for it is written (Mark 16:20): "The Lord working withal, and confirming the word with signs that followed."
On the contrary, Augustine says (83; [Lib. xxi, Sent. sent 4, among the supposititious works of St. Augustine): "Often by means of the magic art miracles are wrought like those which are wrought 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, the demons cannot work miracles, nor can any creature, but God alone: since in the strict sense a miracle is something done outside the order of the entire created nature, under which order every power of a creature is contained. But sometimes miracle may be taken in a wide sense, for whatever exceeds the human power and experience. And thus demons can work miracles, that is, things which rouse man's astonishment, by reason of their being beyond his power and outside his sphere of knowledge. For even a man by doing what is beyond the power and knowledge of another, leads him to marvel at what he has done, so that in a way he seems to that man to have worked a miracle.
It is to be noted, however, that although these works of demons which appear marvelous to us are not real miracles, they are sometimes nevertheless something real. Thus the magicians of Pharaoh by the demons' power produced real serpents and frogs. And "when fire came down from heaven and at one blow consumed Job's servants and sheep; when the storm struck down his house and with it his children--these were the work of Satan, not phantoms"; as Augustine says (De Civ. Dei xx, 19).
Reply to Objection 1. As Augustine says in the same place, the works of Antichrist may be called lying wonders, "either because he will deceive men's senses by means of phantoms, so that he will not really do what he will seem to do; or because, if he work real prodigies, they will lead those into falsehood who believe in him."
Reply to Objection 2. As we have said above (Question 110, Article 2), corporeal matter does not obey either good or bad angels at their will, so that demons be able by their power to transmute matter from one form to another; but they can employ certain seeds that exist in the elements of the world, in order to produce these effects, as Augustine says (De Trin. iii, 8,9). Therefore it must be admitted that all the transformation of corporeal things which can be produced by certain natural powers, to which we must assign the seeds above mentioned, can alike be produced by the operation of the demons, by the employment of these seeds; such as the transformation of certain things into serpents or frogs, which can be produced by putrefaction.
On the contrary, those transformations which cannot be produced by the power of nature, cannot in reality be effected by the operation of the demons; for instance, that the human body be changed into the body of a beast, 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 operation of demons, it is not real but a mere semblance of reality.
Now this may happen in two ways.
Firstly, from within; in this way a demon can work on man's imagination and even on his corporeal senses, so that something seems otherwise that it is, as explained above (111, 3,4). It is said indeed that this can be done sometimes by the power of certain bodies.
Secondly, from without: for just as he can from the air form a body of any form and shape, and assume it so as to appear in it visibly: so, in the same way he can clothe any corporeal thing with any corporeal form, so as to appear therein. This is what Augustine says (De Civ. Dei xviii, 18): "Man's imagination, which whether thinking or dreaming, takes the forms of an innumerable number of things, appears to other men's senses, as it were embodied in the semblance of some animal." This not to be understood as though the imagination itself or the images formed therein were identified with that which appears embodied to the senses of another man: but that the demon, who forms an image in a man's imagination, can offer the same picture to another man's senses.
Reply to Objection 3. As Augustine says (QQ. 83, qu. 79): "When magicians do what holy men do, they do it for a different end and by a different right. The former do it for their own glory; the latter, for the glory of God: the former, by certain private compacts; the latter by the evident assistance and command of God, to Whom every creature is subject."
Source: Summa Theologica (New Advent)