Natural knowledge differs from the knowledge which is by grace. The former remains in the demons, intact, along with their gifts, because, owing to the simplicity of their substance, nothing can be subtracted from their nature. But the knowledge which is by grace is twofold. One kind, which is purely speculative - as the revelation of some Divine secret - is not entirely taken away from them, but as much as is necessary is communicated to them by the angels or other means; but of the other, which is affective and produces love to God, they are wholly deprived; as they are also of charity, which belongs to the Gift of Wisdom.
The will of the demons is confirmed in evil, as that of the other angels is in good, the same operation having taken place in the angels as occurs in us by death, after which there remains no more possibility of meriting; and because the angelic apprehension differs from ours in that it does not make use of discursive reasoning, it apprehends simply and adheres immovably. Thus before adhesion they could freely choose the opposite, but not afterwards; whereas in man the will is flexible in its nature, before election and therefore afterwards.
Fear, grief, joy and other passions of the sensitive appetite which are in the bodily organs do not exist in the demons; nevertheless we must admit such to be in them as simple acts of the will, in which sense grief is nothing else than the struggle of the will against something which is, or is not. Thus the demons wish many things to be which are not, and not to be which are; since they are deprived of the Beatitude which they naturally desire. They desire, also, that some should be lost who are saved, and their evil will is restrained in many ways; which is grievous to them.
The place of the demons is twofold: in hell, absolutely; and in the lower atmosphere until the Day of Judgment; for the probation of mankind, and to give occasion for the ministry of the angels. This is ordered by the Divine Wisdom, which procures the good of inferiors through superiors. Thus, by the good angels we are led directly to good and restrained from evil, while we are guided indirectly by our contest with the bad angels.
And there are demons now in hell, tormenting the souls which they led into sin; as there are good angels with the saints in heaven. Place does not alter their nature, but it affects them with sadness, because they know themselves to be in a place which is not agreeable to their will.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni