Objection 1. It seems that faith alone is the cause of martyrdom. For it is written (1 Peter 4:15-16): "Let none of you suffer as a murderer, or a thief, or a railer, or a coveter of other men's things. But if as a Christian, let him not be ashamed, but let him glorify God in this name." Now a man is said to be a Christian because he holds the faith of Christ. Therefore only faith in Christ gives the glory of martyrdom to those who suffer.
Objection 2. Further, a martyr is a kind of witness. But witness is borne to the truth alone. Now one is not called a martyr for bearing witness to any truth, but only for witnessing to the Divine truth, otherwise a man would be a martyr if he were to die for confessing a truth of geometry or some other speculative science, which seems ridiculous. Therefore faith alone is the cause of martyrdom.
Objection 3. Further, those virtuous deeds would seem to be of most account which are directed to the common good, since "the good of the nation is better than the good of the individual," according to the Philosopher (Ethic. i, 2). If, then, some other good were the cause of martyrdom, it would seem that before all those would be martyrs who die for the defense of their country. Yet this is not consistent with Church observance, for we do not celebrate the martyrdom of those who die in a just war. Therefore faith alone is the cause of martyrdom.
On the contrary, It is written (Matthew 5:10): "Blessed are they that suffer persecution for justice' sake," which pertains to martyrdom, according to a gloss, as well as Jerome's commentary on this passage. Now not only faith but also the other virtues pertain to justice. Therefore other virtues can be the cause of martyrdom.
I answer that, As stated above (Article 4), martyrs are so called as being witnesses, because by suffering in body unto death they bear witness to the truth; not indeed to any truth, but to the truth which is in accordance with godliness, and was made known to us by Christ: wherefore Christ's martyrs are His witnesses. Now this truth is the truth of faith. Wherefore the cause of all martyrdom is the truth of faith.
But the truth of faith includes not only inward belief, but also outward profession, which is expressed not only by words, whereby one confesses the faith, but also by deeds, whereby a person shows that he has faith, according to James 2:18, "I will show thee, by works, my faith." Hence it is written of certain people (Titus 1:16): "They profess that they know God but in their works they deny Him." Thus all virtuous deeds, inasmuch as they are referred to God, are professions of the faith whereby we come to know that God requires these works of us, and rewards us for them: and in this way they can be the cause of martyrdom. For this reason the Church celebrates the martyrdom of Blessed John the Baptist, who suffered death, not for refusing to deny the faith, but for reproving adultery.
Reply to Objection 1. A Christian is one who is Christ's. Now a person is said to be Christ's, not only through having faith in Christ, but also because he is actuated to virtuous deeds by the Spirit of Christ, according to Romans 8:9, "If any man have not the Spirit of Christ, he is none of His"; and again because in imitation of Christ he is dead to sins, according to Galatians 5:24, "They that are Christ's have crucified their flesh with the vices and concupiscences." Hence to suffer as a Christian is not only to suffer in confession of the faith, which is done by words, but also to suffer for doing any good work, or for avoiding any sin, for Christ's sake, because this all comes under the head of witnessing to the faith.
Reply to Objection 2. The truth of other sciences has no connection with the worship of the Godhead: hence it is not called truth according to godliness, and consequently the confession thereof cannot be said to be the direct cause of martyrdom. Yet, since every lie is a sin, as stated above (110, 3,4), avoidance of a lie, to whatever truth it may be contrary, may be the cause of martyrdom inasmuch as a lie is a sin against the Divine Law.
Reply to Objection 3. The good of one's country is paramount among human goods: yet the Divine good, which is the proper cause of martyrdom, is of more account than human good. Nevertheless, since human good may become Divine, for instance when it is referred to God, it follows that any human good in so far as it is referred to God, may be the cause of martyrdom.
Source: Summa Theologica (New Advent)