Can demons repent?
In the Thomistic tradition, they cannot; they will not. We can choose to repent, the demons do not. It is not a choice they will ever, ever make. The angels are from eternity, and beyond all time. Unlike ourselves, they know the truth without room for doubt. They made their decision in the very instant of their existence.
THE CONSEQUENCES OF THIS SINGLE MOMENT of trial of the Angels are staggering. There is no such thing as a second chance for an Angel, no period of contrition and penance. Their freedom from ignorance and passion, their instantaneous grasp of truth removes all possibility of a change of will for them. They love or hate at once and beyond recall; as fixed by that instant as we are by death.................
THE EVIL ANGELS, in that first instant of their abuse of liberty, rejected God. Caught in a deliberate fascination of their own beauty, they refused to look to that beauty's source, refused to seek for happiness outside their own satisfying self; and so attempted to find in themselves what can be found only in God-----the answer to the will's Divinely given desire for goodness without limit. These devils can now sin all they like, and know themselves less free with every sin; the abuse of liberty mounts with each sin, the chains grow more galling, the self-imposed slavery more bitter, and the hatred more consumingly intense. Their choice was freely made, abusing liberty; and it is eternally confirmed to make up Hell's most despairing torment.
The decisions made by eternal spirit creatures are eternal and irrevocable. They have chosen pride and envy, and their choice is eternal; they will never change their minds. We, who are so much less than the fallen angels, will yet be greater than them, and this torments them and makes them hate us all the more.
THAT A MERE MAN, the lowest of intellectual creatures and so far beneath the devils in natural gifts, should, by the grace of God, go beyond the limits of nature to eternal life in the home of God is galling to the devil and a constant prod to his envy. That this particular soul should reach such heights triumphing over Satan's diabolic genius is a bitter humiliation and added fuel to the fire of his hatred of God. Both that envy and hatred are fed by the devil's disgust with the sins of men. True, he knows that he is guilty of all the sins he induces men to commit; but that guilt is a far cry from any affection for the things that so easily enslave a man. A man surrendering to the allure or violence of passion, immersing himself in the world of sense, playing the slave to things designed to serve him-----all this is revolting to the devil's purely spiritual nature even when he is playing the principal part in bringing about such a degradation of a man. His utter disgust with the depths to which man can sink is still more reason for his envy that such creatures can still aspire to heaven while Satan himself must grovel eternally in Hell.
Our wills, whether human or angelic, are sovereign, and not even God will violate them. We humans live in the span of Time, and our choices can still change. The demons then chose to reject God, now choose to reject God, and will always choose to reject God. Forever. Their choice is their Hell and torment.
BTS