Objection 1. It would seem that those miracles were unfitting which Christ worked in spiritual substances. For among spiritual substances the holy angels are above the demons; for, as Augustine says (De Trin. ii.): "The treacherous and sinful rational spirit of life is ruled by the rational, pious, and just spirit of life." But we read of no miracles worked by Christ in the good angels. Therefore neither should He have worked miracles in the demons.
Objection 2. Further, Christ's miracles were ordained to make known His Godhead. But Christ's Godhead was not to be made known to the demons: since this would have hindered the mystery of His Passion, according to 1 Corinthians 2:8: "If they had known it, they would never have crucified the Lord of glory." Therefore He should not have worked miracles in the demons.
Objection 3. Further, Christ's miracles were ordained to the glory of God: hence it is written (Matthew 9:8) that "the multitudes seeing" that the man sick of the palsy had been healed by Christ, "feared, and glorified God that gave such power to men." But the demons have no part in glorifying God; since "praise is not seemly in the mouth of a sinner" (Sirach 15:9). For which reason also "He suffered them not to speak" (Mark 1:34; Luke 4:41) those things which reflected glory on Him. Therefore it seems that it was unfitting for Him to work miracles in the demons.
Objection 4. Further, Christ's miracles are ordained to the salvation of mankind. But sometimes the casting out of demons from men was detrimental to man, in some cases to the body: thus it is related (Mark 9:24-25) that a demon at Christ's command, "crying out and greatly tearing" the man, "went out of him; and he became as dead, so that many said: He is dead"; sometimes also to things: as when He sent the demons, at their own request, into the swine, which they cast headlong into the sea; wherefore the inhabitants of those parts "besought Him that He would depart from their coasts" (Matthew 8:31-34). Therefore it seems unfitting that He should have worked such like miracles.
On the contrary, this was foretold (Zechariah 13:2), where it is written: "I will take away... the unclean spirit out of the earth."
I answer that, The miracles worked by Christ were arguments for the faith which He taught. Now, by the power of His Godhead He was to rescue those who would believe in Him, from the power of the demons; according to John 12:31: "Now shall the prince of this world be cast out." Consequently it was fitting that, among other miracles, He should also deliver those who were obsessed by demons.
Reply to Objection 1. Just as men were to be delivered by Christ from the power of the demons, so by Him were they to be brought to the companionship of the angels, according to Colossians 1:20: "Making peace through the blood of His cross, both as to the things on earth and the things that are in heaven." Therefore it was not fitting to show forth to men other miracles as regards the angels, except by angels appearing to men: as happened in His Nativity, His Resurrection, and His Ascension.
Reply to Objection 2. As Augustine says (De Civ. Dei ix): "Christ was known to the demons just as much as He willed; and He willed just as far as there was need. But He was known to them, not as to the holy angels, by that which is eternal life, but by certain temporal effects of His power." First, when they saw that Christ was hungry after fasting they deemed Him not to be the Son of God. Hence, on Luke 4:3, "If Thou be the Son of God," etc., Ambrose says: "What means this way of addressing Him? save that, though He knew that the Son of God was to come, yet he did not think that He had come in the weakness of the flesh?" But afterwards, when he saw Him work miracles, he had a sort of conjectural suspicion that He was the Son of God. Hence on Mark 1:24, "I know who Thou art, the Holy one of God," Chrysostom [Victor of Antioch. Cf. Catena Aurea] says that "he had no certain or firm knowledge of God's coming." Yet he knew that He was "the Christ promised in the Law," wherefore it is said (Luke 4:41) that "they knew that He was Christ." But it was rather from suspicion than from certainty that they confessed Him to be the Son of God. Hence Bede says on Luke 4:41: "The demons confess the Son of God, and, as stated farther on, 'they knew that He was Christ.' For when the devil saw Him weakened by His fast, He knew Him to be a real man: but when He failed to overcome Him by temptation, He doubted lest He should be the Son of God. And now from the power of His miracles He either knew, or rather suspected that He was the Son of God. His reason therefore for persuading the Jews to crucify Him was not that he deemed Him not to be Christ or the Son of God, but because he did not foresee that he would be the loser by His death. For the Apostle says of this mystery" (1 Corinthians 2:7-8), "which is hidden from the beginning, that 'none of the princes of this world knew it,' for if they had known it they would never have crucified the Lord of glory."
Reply to Objection 3. The miracles which Christ worked in expelling demons were for the benefit, not of the demons, but of men, that they might glorify Him. Wherefore He forbade them to speak in His praise. First, to give us an example. For, as Athanasius says, "He restrained his speech, although he was confessing the truth; to teach us not to care about such things, although it may seem that what is said is true. For it is wrong to seek to learn from the devil when we have the Divine Scripture": Besides, it is dangerous, since the demons frequently mix falsehood with truth. Or, as Chrysostom [Cyril of Alexandria, Comment. in Luc.] says: "It was not meet for them to usurp the prerogative of the apostolic office. Nor was it fitting that the mystery of Christ should be proclaimed by a corrupt tongue" because "praise is not seemly in the mouth of a sinner" [Cf. Theophylact, Enarr. in Luc.]. Thirdly, because, as Bede says, "He did not wish the envy of the Jews to be aroused thereby" [Bede, Expos. in Luc. iv, 41]. Hence "even the apostles are commanded to be silent about Him, lest, if His Divine majesty were proclaimed, the gift of His Passion should be deferred."
Reply to Objection 4. Christ came specially to teach and to work miracles for the good of man, and principally as to the salvation of his soul. Consequently, He allowed the demons, that He cast out, to do man some harm, either in his body or in his goods, for the salvation of man's soul--namely, for man's instruction. Hence Chrysostom says on Matthew 8:32 that Christ let the demons depart into the swine, "not as yielding to the demons, but first, to show... how harmful are the demons who attack men; secondly, that all might learn that the demons would not dare to hurt even the swine, except He allow them; thirdly, that they would have treated those men more grievously than they treated the swine, unless they had been protected by God's providence."
And for the same motives He allowed the man, who was being delivered from the demons, to suffer grievously for the moment; yet did He release him at once from that distress. By this, moreover, we are taught, as Bede says on Mark 9:25, that "often, when after falling into sin we strive to return to God, we experience further and more grievous attacks from the old enemy. This he does, either that he may inspire us with a distaste for virtue, or that he may avenge the shame of having been cast out." For the man who was healed "became as dead," says Jerome, "because to those who are healed it is said, 'You are dead; and your life is hid with Christ in God'" (Colossians 3:3)
Source: Summa Theologica (New Advent)