Objection 1. It would seem that the gifts of the Holy Ghost are not habits. Because a habit is a quality abiding in man, being defined as "a quality difficult to remove," as stated in the Predicaments (Categor. vi). Now it is proper to Christ that the gifts of the Holy Ghost rest in Him, as stated in Isaiah 11:2-3: "He upon Whom thou shalt see the Spirit descending and remaining upon Him, He it is that baptizeth"; on which words Gregory comments as follows (Moral. ii, 27): "The Holy Ghost comes upon all the faithful; but, in a singular way, He dwells always in the Mediator." Therefore the gifts of the Holy Ghost are not habits.
Objection 2. Further, the gifts of the Holy Ghost perfect man according as he is moved by the Spirit of God, as stated above (1,2). But in so far as man is moved by the Spirit of God, he is somewhat like an instrument in His regard. Now to be perfected by a habit is befitting, not an instrument, but a principal agent. Therefore the gifts of the Holy Ghost are not habits.
Objection 3. Further, as the gifts of the Holy Ghost are due to Divine inspiration, so is the gift of prophecy. Now prophecy is not a habit: for "the spirit of prophecy does not always reside in the prophets," as Gregory states (Hom. i in Ezechiel). Neither, therefore, are the gifts of the Holy Ghost.
On the contrary, Our Lord in speaking of the Holy Ghost said to His disciples (John 14:17): "He shall abide with you, and shall be in you." Now the Holy Ghost is not in a man without His gifts. Therefore His gifts abide in man. Therefore they are not merely acts or passions but abiding habits.
I answer that, As stated above (Article 1), the gifts are perfections of man, whereby he becomes amenable to the promptings of the Holy Ghost. Now it is evident from what has been already said (56, 4; 58, 2), that the moral virtues perfect the appetitive power according as it partakes somewhat of the reason, in so far, to wit, as it has a natural aptitude to be moved by the command of reason. Accordingly the gifts of the Holy Ghost, as compared with the Holy Ghost Himself, are related to man, even as the moral virtues, in comparison with the reason, are related to the appetitive power. Now the moral virtues are habits, whereby the powers of appetite are disposed to obey reason promptly. Therefore the gifts of the Holy Ghost are habits whereby man is perfected to obey readily the Holy Ghost.
Reply to Objection 1. Gregory solves this objection (Moral. ii, 27) by saying that "by those gifts without which one cannot obtain life, the Holy Ghost ever abides in all the elect, but not by His other gifts." Now the seven gifts are necessary for salvation, as stated above (Article 2). Therefore, with regard to them, the Holy Ghost ever abides in holy men.
Reply to Objection 2. This argument holds, in the case of an instrument which has no faculty of action, but only of being acted upon. But man is not an instrument of that kind; for he is so acted upon, by the Holy Ghost, that he also acts himself, in so far as he has a free-will. Therefore he needs a habit.
Reply to Objection 3. Prophecy is one of those gifts which are for the manifestation of the Spirit, not for the necessity of salvation: hence the comparison fails.
Source: Summa Theologica (New Advent)