Objection 1. It seems that confidence does not belong to magnanimity. For a man may have assurance not only in himself, but also in another, according to 2 Corinthians 3:4-5, "Such confidence we have, through Christ towards God, not that we are sufficient to think anything of ourselves, as of ourselves." But this seems inconsistent with the idea of magnanimity. Therefore confidence does not belong to magnanimity.
Objection 2. Further, confidence seems to be opposed to fear, according to Isaiah 12:2, "I will deal confidently and will not fear." But to be without fear seems more akin to fortitude. Therefore confidence also belongs to fortitude rather than to magnanimity.
Objection 3. Further, reward is not due except to virtue. But a reward is due to confidence, according to Hebrews 3:6, where it is said that we are the house of Christ, "if we hold fast the confidence and glory of hope unto the end." Therefore confidence is a virtue distinct from magnanimity: and this is confirmed by the fact that Macrobius enumerates it with magnanimity (In Somn. Scip. i).
On the contrary, Tully (De Suv. Rhet. ii) seems to substitute confidence for magnanimity, as stated above in the preceding Question (ad 6) and in the prologue to this.
I answer that, Confidence takes its name from "fides" [faith]: and it belongs to faith to believe something and in somebody. But confidence belongs to hope, according to Job 11:18, "Thou shalt have confidence, hope being set before thee." Wherefore confidence apparently denotes chiefly that a man derives hope through believing the word of one who promises to help him. Since, however, faith signifies also a strong opinion, and since one may come to have a strong opinion about something, not only on account of another's statement, but also on account of something we observe in another, it follows that confidence may denote the hope of having something, which hope we conceive through observing something either in oneself--for instance, through observing that he is healthy, a man is confident that he will live long. or in another, for instance, through observing that another is friendly to him and powerful, a man is confident that he will receive help from him.
Now it has been stated above (1, ad 2) that magnanimity is chiefly about the hope of something difficult. Wherefore, since confidence denotes a certain strength of hope arising from some observation which gives one a strong opinion that one will obtain a certain good, it follows that confidence belongs to magnanimity.
Reply to Objection 1. As the Philosopher says (Ethic. iv, 3), it belongs to the "magnanimous to need nothing," for need is a mark of the deficient. But this is to be understood according to the mode of a man, hence he adds "or scarcely anything." For it surpasses man to need nothing at all. For every man needs, first, the Divine assistance, secondly, even human assistance, since man is naturally a social animal, for he is sufficient by himself to provide for his own life. Accordingly, in so far as he needs others, it belongs to a magnanimous man to have confidence in others, for it is also a point of excellence in a man that he should have at hand those who are able to be of service to him. And in so far as his own ability goes, it belongs to a magnanimous man to be confident in himself.
Reply to Objection 2. As stated above (I-II, 23, 2; I-II, 40, 4), when we were treating of the passions, hope is directly opposed to despair, because the latter is about the same object, namely good. But as regards contrariety of objects it is opposed to fear, because the latter's object is evil. Now confidence denotes a certain strength of hope, wherefore it is opposed to fear even as hope is. Since, however, fortitude properly strengthens a man in respect of evil, and magnanimity in respect of the obtaining of good, it follows that confidence belongs more properly to magnanimity than to fortitude. Yet because hope causes daring, which belongs to fortitude, it follows in consequence that confidence pertains to fortitude.
Reply to Objection 3. Confidence, as stated above, denotes a certain mode of hope: for confidence is hope strengthened by a strong opinion. Now the mode applied to an affection may call for commendation of the act, so that it become meritorious, yet it is not this that draws it to a species of virtue, but its matter. Hence, properly speaking, confidence cannot denote a virtue, though it may denote the conditions of a virtue. For this reason it is reckoned among the parts of fortitude, not as an annexed virtue, except as identified with magnanimity by Tully (De Suv. Rhet. ii), but as an integral part, as stated in the preceding Question.
Source: Summa Theologica (New Advent)