Objection 1. It would seem that the gift of understanding is not in all who are in a state of grace. For Gregory says (Moral. ii, 49) that "the gift of understanding is given as a remedy against dulness of mind." Now many who are in a state of grace suffer from dulness of mind. Therefore the gift of understanding is not in all who are in a state of grace.
Objection 2. Further, of all the things that are connected with knowledge, faith alone seems to be necessary for salvation, since by faith Christ dwells in our hearts, according to Ephesians 3:17. Now the gift of understanding is not in everyone that has faith; indeed, those who have faith ought to pray that they may understand, as Augustine says (De Trin. xv, 27). Therefore the gift of understanding is not necessary for salvation: and, consequently, is not in all who are in a state of grace.
Objection 3. Further, those things which are common to all who are in a state of grace, are never withdrawn from them. Now the grace of understanding and of the other gifts sometimes withdraws itself profitably, for, at times, "when the mind is puffed up with understanding sublime things, it becomes sluggish and dull in base and vile things," as Gregory observes (Moral. ii, 49). Therefore the gift of understanding is not in all who are in a state of grace.
On the contrary, It is written (Psalm 81:5): "They have not known or understood, they walk on in darkness." But no one who is in a state of grace walks in darkness, according to John 8:12: "He that followeth Me, walketh not in darkness." Therefore no one who is in a state of grace is without the gift of understanding.
I answer that, In all who are in a state of grace, there must needs be rectitude of the will, since grace prepares man's will for good, according to Augustine (Contra Julian. Pelag. iv, 3). Now the will cannot be rightly directed to good, unless there be already some knowledge of the truth, since the object of the will is good understood, as stated in De Anima iii, 7. Again, just as the Holy Ghost directs man's will by the gift of charity, so as to move it directly to some supernatural good; so also, by the gift of understanding, He enlightens the human mind, so that it knows some supernatural truth, to which the right will needs to tend.
Therefore, just as the gift of charity is in all of those who have sanctifying grace, so also is the gift of understanding.
Reply to Objection 1. Some who have sanctifying grace may suffer dulness of mind with regard to things that are not necessary for salvation; but with regard to those that are necessary for salvation, they are sufficiently instructed by the Holy Ghost, according to 1 John 2:27: "His unction teacheth you of all things."
Reply to Objection 2. Although not all who have faith understand fully the things that are proposed to be believed, yet they understand that they ought to believe them, and that they ought nowise to deviate from them.
Reply to Objection 3. With regard to things necessary for salvation, the gift of understanding never withdraws from holy persons: but, in order that they may have no incentive to pride, it does withdraw sometimes with regard to other things, so that their mind is unable to penetrate all things clearly.
Source: Summa Theologica (New Advent)