Objection 1. It would seem that the will of the demons is not obstinate in evil. For liberty of will belongs to the nature of an intellectual being, which nature remains in the demons, as we said above (Article 1). But liberty of will is directly and firstly ordained to good rather than to evil. Therefore the demons' will is not so obstinate in evil as not to be able to return to what is good.
Objection 2. Further, since God's mercy is infinite, it is greater than the demons' malice, which is finite. But no one returns from the malice of sin to the goodness of justice save through God's mercy. Therefore the demons can likewise return from their state of malice to the state of justice.
Objection 3. Further, if the demons have a will obstinate in evil, then their will would be especially obstinate in the sin whereby they fell. But that sin, namely, pride, is in them no longer; because the motive for the sin no longer endures, namely, excellence. Therefore the demon is not obstinate in malice.
Objection 4. Further, Gregory says (Moral. iv) that man can be reinstated by another, since he fell through another. But, as was observed already (63, 8), the lower demons fell through the highest one. Therefore their fall can be repaired by another. Consequently they are not obstinate in malice.
Objection 5. Further, whoever is obstinate in malice, never performs any good work. But the demon performs some good works: for he confesses the truth, saying to Christ: "I know Who Thou art, the holy one of God" (Mark 1:24). "The demons" also "believe and tremble" (James 2:19). And Dionysius observes (Div. Nom. iv), that "they desire what is good and best, which is, to be, to live, to understand." Therefore they are not obstinate in malice.
On the contrary, It is said (Psalm 73:23): "The pride of them that hate Thee, ascendeth continually"; and this is understood of the demons. Therefore they remain ever obstinate in their malice.
I answer that, It was Origen's opinion [Peri Archon i. 6 that every will of the creature can by reason of free-will be inclined to good and evil; with the exception of the soul of Christ on account of the union of the Word. Such a statement deprives angels and saints of true beatitude, because everlasting stability is of the very nature of true beatitude; hence it is termed "life everlasting." It is also contrary to the authority of Sacred Scripture, which declares that demons and wicked men shall be sent "into everlasting punishment," and the good brought "into everlasting life." Consequently such an opinion must be considered erroneous; while according to Catholic Faith, it must be held firmly both that the will of the good angels is confirmed in good, and that the will of the demons is obstinate in evil.
We must seek for the cause of this obstinacy, not in the gravity of the sin, but in the condition of their nature or state. For as Damascene says (De Fide Orth. ii), "death is to men, what the fall is to the angels." Now it is clear that all the mortal sins of men, grave or less grave, are pardonable before death; whereas after death they are without remission and endure for ever.
To find the cause, then, of this obstinacy, it must be borne in mind that the appetitive power is in all things proportioned to the apprehensive, whereby it is moved, as the movable by its mover. For the sensitive appetite seeks a particular good; while the will seeks the universal good, as was said above (Question 59, Article 1); as also the sense apprehends particular objects, while the intellect considers universals. Now the angel's apprehension differs from man's in this respect, that the angel by his intellect apprehends immovably, as we apprehend immovably first principles which are the object of the habit of "intelligence"; whereas man by his reason apprehends movably, passing from one consideration to another; and having the way open by which he may proceed to either of two opposites. Consequently man's will adheres to a thing movably, and with the power of forsaking it and of clinging to the opposite; whereas the angel's will adheres fixedly and immovably. Therefore, if his will be considered before its adhesion, it can freely adhere either to this or to its opposite (namely, in such things as he does not will naturally); but after he has once adhered, he clings immovably. So it is customary to say that man's free-will is flexible to the opposite both before and after choice; but the angel's free-will is flexible either opposite before the choice, but not after. Therefore the good angels who adhered to justice, were confirmed therein; whereas the wicked ones, sinning, are obstinate in sin. Later on we shall treat of the obstinacy of men who are damned (SP, 98, 1, 2).
Reply to Objection 1. The good and wicked angels have free-will, but according to the manner and condition of their state, as has been said.
Reply to Objection 2. God's mercy delivers from sin those who repent. But such as are not capable of repenting, cling immovably to sin, and are not delivered by the Divine mercy.
Reply to Objection 3. The devil's first sin still remains in him according to desire; although not as to his believing that he can obtain what he desired. Even so, if a man were to believe that he can commit murder, and wills to commit it, and afterwards the power is taken from him; nevertheless, the will to murder can stay with him, so that he would he had done it, or still would do it if he could.
Reply to Objection 4. The fact that man sinned from another's suggestion, is not the whole cause of man's sin being pardonable. Consequently the argument does not hold good.
Reply to Objection 5. A demon's act is twofold. One comes of deliberate will; and this is properly called his own act. Such an act on the demon's part is always wicked; because, although at times he does something good, yet he does not do it well; as when he tells the truth in order to deceive; and when he believes and confesses, yet not willingly, but compelled by the evidence of things. Another kind of act is natural to the demon; this can be good and bears witness to the goodness of nature. Yet he abuses even such good acts to evil purpose.
Source: Summa Theologica (New Advent)