Objection 1. It would seem that the gift of understanding is found also in those who have not sanctifying grace. For Augustine, in expounding the words of Psalm 118:20: "My soul hath coveted to long for Thy justifications," says: "Understanding flies ahead, and man's will is weak and slow to follow." But in all who have sanctifying grace, the will is prompt on account of charity. Therefore the gift of understanding can be in those who have not sanctifying grace.
Objection 2. Further, it is written (Daniel 10:1) that "there is need of understanding in a" prophetic "vision," so that, seemingly, there is no prophecy without the gift of understanding. But there can be prophecy without sanctifying grace, as evidenced by Matthew 7:22, where those who say: "We have prophesied in Thy name [Vulgate: 'Have we not prophesied in Thy name?]," are answered with the words: "I never knew you." Therefore the gift of understanding can be without sanctifying grace.
Objection 3. Further, the gift of understanding responds to the virtue of faith, according to Isaiah 7:9, following another reading [the Septuagint]: "If you will not believe you shall not understand." Now faith can be without sanctifying grace. Therefore the gift of understanding can be without it.
On the contrary, Our Lord said (John 6:45): "Every one that hath heard of the Father, and hath learned, cometh to Me." Now it is by the intellect, as Gregory observes (Moral. i, 32), that we learn or understand what we hear. Therefore whoever has the gift of understanding, cometh to Christ, which is impossible without sanctifying grace. Therefore the gift of understanding cannot be without sanctifying grace.
I answer that, As stated above (I-II, 68, 1,2) the gifts of the Holy Ghost perfect the soul, according as it is amenable to the motion of the Holy Ghost. Accordingly then, the intellectual light of grace is called the gift of understanding, in so far as man's understanding is easily moved by the Holy Ghost, the consideration of which movement depends on a true apprehension of the end. Wherefore unless the human intellect be moved by the Holy Ghost so far as to have a right estimate of the end, it has not yet obtained the gift of understanding, however much the Holy Ghost may have enlightened it in regard to other truths that are preambles to the faith.
Now to have a right estimate about the last end one must not be in error about the end, and must adhere to it firmly as to the greatest good: and no one can do this without sanctifying grace; even as in moral matters a man has a right estimate about the end through a habit of virtue. Therefore no one has the gift of understanding without sanctifying grace.
Reply to Objection 1. By understanding Augustine means any kind of intellectual light, that, however, does not fulfil all the conditions of a gift, unless the mind of man be so far perfected as to have a right estimate about the end.
Reply to Objection 2. The understanding that is requisite for prophecy, is a kind of enlightenment of the mind with regard to the things revealed to the prophet: but it is not an enlightenment of the mind with regard to a right estimate about the last end, which belongs to the gift of understanding.
Reply to Objection 3. Faith implies merely assent to what is proposed but understanding implies a certain perception of the truth, which perception, except in one who has sanctifying grace, cannot regard the end, as stated above. Hence the comparison fails between understanding and faith.
Source: Summa Theologica (New Advent)