Objection 1. It seems that martyrdom is not an act of fortitude. For the Greek martyr signifies a witness. Now witness is borne to the faith of Christ. according to Acts 1:8, "You shall be witnesses unto Me," etc. and Maximus says in a sermon: "The mother of martyrs is the Catholic faith which those glorious warriors have sealed with their blood." Therefore martyrdom is an act of faith rather than of fortitude.
Objection 2. Further, a praiseworthy act belongs chiefly to the virtue which inclines thereto, is manifested thereby, and without which the act avails nothing. Now charity is the chief incentive to martyrdom: Thus Maximus says in a sermon: "The charity of Christ is victorious in His martyrs." Again the greatest proof of charity lies in the act of martyrdom, according to John 15:13, "Greater love than this no man hath, that a man lay down his life for his friends." Moreover without charity martyrdom avails nothing, according to 1 Corinthians 13:3, "If I should deliver my body to be burned, and have not charity, it profiteth me nothing." Therefore martyrdom is an act of charity rather than of fortitude.
Objection 3. Further, Augustine says in a sermon on St. Cyprian: "It is easy to honor a martyr by singing his praises, but it is a great thing to imitate his faith and patience." Now that which calls chiefly for praise in a virtuous act, is the virtue of which it is the act. Therefore martyrdom is an act of patience rather than of fortitude.
On the contrary, Cyprian says (Ep. ad Mart. et Conf. ii): "Blessed martyrs, with what praise shall I extol you? Most valiant warriors, how shall I find words to proclaim the strength of your courage?" Now a person is praised on account of the virtue whose act he performs. Therefore martyrdom is an act of fortitude.
I answer that, As stated above (123, 1, seqq.), it belongs to fortitude to strengthen man in the good of virtue, especially against dangers, and chiefly against dangers of death, and most of all against those that occur in battle. Now it is evident that in martyrdom man is firmly strengthened in the good of virtue, since he cleaves to faith and justice notwithstanding the threatening danger of death, the imminence of which is moreover due to a kind of particular contest with his persecutors. Hence Cyprian says in a sermon (Ep. ad Mart. et Conf. ii): "The crowd of onlookers wondered to see an unearthly battle, and Christ's servants fighting erect, undaunted in speech, with souls unmoved, and strength divine." Wherefore it is evident that martyrdom is an act of fortitude; for which reason the Church reads in the office of Martyrs: They "became valiant in battle" [Hebrews 11:34.
Reply to Objection 1. Two things must be considered in the act of fortitude. one is the good wherein the brave man is strengthened, and this is the end of fortitude; the other is the firmness itself, whereby a man does not yield to the contraries that hinder him from achieving that good, and in this consists the essence of fortitude. Now just as civic fortitude strengthens a man's mind in human justice, for the safeguarding of which he braves the danger of death, so gratuitous fortitude strengthens man's soul in the good of Divine justice, which is "through faith in Christ Jesus," according to Romans 3:22. Thus martyrdom is related to faith as the end in which one is strengthened, but to fortitude as the eliciting habit.
Reply to Objection 2. Charity inclines one to the act of martyrdom, as its first and chief motive cause, being the virtue commanding it, whereas fortitude inclines thereto as being its proper motive cause, being the virtue that elicits it. Hence martyrdom is an act of charity as commanding, and of fortitude as eliciting. For this reason also it manifests both virtues. It is due to charity that it is meritorious, like any other act of virtue: and for this reason it avails not without charity.
Reply to Objection 3. As stated above (Question 123, Article 6), the chief act of fortitude is endurance: to this and not to its secondary act, which is aggression, martyrdom belongs. And since patience serves fortitude on the part of its chief act, viz. endurance, hence it is that martyrs are also praised for their patience.
Source: Summa Theologica (New Advent)