Objection 1. It would seem that, among the fruits, faith does not respond to the gift of understanding. For understanding is the fruit of faith, since it is written (Isaiah 7:9) according to another reading [the Septuagint]: "If you will not believe you shall not understand," where our version has: "If you will not believe, you shall not continue." Therefore fruit is not the fruit of understanding.
Objection 2. Further, that which precedes is not the fruit of what follows.
But faith seems to precede understanding, since it is the foundation of the entire spiritual edifice, as stated above (4, 1,7). Therefore faith is not the fruit of understanding.
Objection 3. Further, more gifts pertain to the intellect than to the appetite. Now, among the fruits, only one pertains to the intellect; namely, faith, while all the others pertain to the appetite. Therefore faith, seemingly, does not pertain to understanding more than to wisdom, knowledge or counsel.
On the contrary, The end of a thing is its fruit. Now the gift of understanding seems to be ordained chiefly to the certitude of faith, which certitude is reckoned a fruit. For a gloss on Galatians 5:22 says that the "faith which is a fruit, is certitude about the unseen." Therefore faith, among the fruits, responds to the gift of understanding.
I answer that, The fruits of the Spirit, as stated above (I-II, 70, 1), when we were discussing them, are so called because they are something ultimate and delightful, produced in us by the power of the Holy Ghost. Now the ultimate and delightful has the nature of an end, which is the proper object of the will: and consequently that which is ultimate and delightful with regard to the will, must be, after a fashion, the fruit of all the other things that pertain to the other powers.
Accordingly, therefore, to this kind of gift of virtue that perfects a power, we may distinguish a double fruit: one, belonging to the same power; the other, the last of all as it were, belonging to the will. On this way we must conclude that the fruit which properly responds to the gift of understanding is faith, i.e. the certitude of faith; while the fruit that responds to it last of all is joy, which belongs to the will.
Reply to Objection 1. Understanding is the fruit of faith, taken as a virtue. But we are not taking faith in this sense here, but for a kind of certitude of faith, to which man attains by the gift of understanding.
Reply to Objection 2. Faith cannot altogether precede understanding, for it would be impossible to assent by believing what is proposed to be believed, without understanding it in some way. However, the perfection of understanding follows the virtue of faith: which perfection of understanding is itself followed by a kind of certainty of faith.
Reply to Objection 3. The fruit of practical knowledge cannot consist in that very knowledge, since knowledge of that kind is known not for its own sake, but for the sake of something else. On the other hand, speculative knowledge has its fruit in its very self, which fruit is the certitude about the thing known. Hence the gift of counsel, which belongs only to practical knowledge, has no corresponding fruit of its own: while the gifts of wisdom, understanding and knowledge, which can belongs also to speculative knowledge, have but one corresponding fruit, which is certainly denoted by the name of faith. The reason why there are several fruits pertaining to the appetitive faculty, is because, as already stated, the character of end, which the word fruit implies, pertains to the appetitive rather than to the intellective part.
Source: Summa Theologica (New Advent)