Objection 1. It seems that fearlessness is not a sin. For that which is reckoned to the praise of a just man is not a sin. Now it is written in praise of the just man (Proverbs 28:1): "The just, bold as a lion, shall be without dread." Therefore it is not a sin to be without fear.
Objection 2. Further, nothing is so fearful as death, according to the Philosopher (Ethic. iii, 6). Yet one ought not to fear even death, according to Matthew 10:28, "Fear ye not them that kill the body," etc., nor anything that can be inflicted by man, according to Isaiah 51:12, "Who art thou, that thou shouldst be afraid of a mortal man?" Therefore it is not a sin to be fearless.
Objection 3. Further, fear is born of love, as stated above (Question 125, Article 2). Now it belongs to the perfection of virtue to love nothing earthly, since according to Augustine (De Civ. Dei xiv), "the love of God to the abasement of self makes us citizens of the heavenly city." Therefore it is seemingly not a sin to fear nothing earthly.
On the contrary, It is said of the unjust judge (Luke 18:2) that "he feared not God nor regarded man."
I answer that, Since fear is born of love, we must seemingly judge alike of love and fear. Now it is here a question of that fear whereby one dreads temporal evils, and which results from the love of temporal goods. And every man has it instilled in him by nature to love his own life and whatever is directed thereto; and to do so in due measure, that is, to love these things not as placing his end therein, but as things to be used for the sake of his last end. Hence it is contrary to the natural inclination, and therefore a sin, to fall short of loving them in due measure. Nevertheless, one never lapses entirely from this love: since what is natural cannot be wholly lost: for which reason the Apostle says (Ephesians 5:29): "No man ever hated his own flesh." Wherefore even those that slay themselves do so from love of their own flesh, which they desire to free from present stress. Hence it may happen that a man fears death and other temporal evils less than he ought, for the reason that he loves them* less than he ought. [Viz. the contrary goods. One would expect 'se' instead of 'ea.' We should then read: For the reason that he loves himself less than he ought.] But that he fear none of these things cannot result from an entire lack of love, but only from the fact that he thinks it impossible for him to be afflicted by the evils contrary to the goods he loves. This is sometimes the result of pride of soul presuming on self and despising others, according to the saying of Job 41:24-25: "He [Vulgate: 'who'] was made to fear no one, he beholdeth every high thing": and sometimes it happens through a defect in the reason; thus the Philosopher says (Ethic. iii, 7) that the "Celts, through lack of intelligence, fear nothing." ["A man would deserve to be called insane and senseless if there were nothing that he feared, not even an earthquake nor a storm at sea, as is said to be the case with the Celts."] It is therefore evident that fearlessness is a vice, whether it result from lack of love, pride of soul, or dullness of understanding: yet the latter is excused from sin if it be invincible.
Reply to Objection 1. The just man is praised for being without fear that withdraws him from good; not that he is altogether fearless, for it is written (Sirach 1:28): "He that is without fear cannot be justified."
Reply to Objection 2. Death and whatever else can be inflicted by mortal man are not to be feared so that they make us forsake justice: but they are to be feared as hindering man in acts of virtue, either as regards himself, or as regards the progress he may cause in others. Hence it is written (Proverbs 14:16): "A wise man feareth and declineth from evil."
Reply to Objection 3. Temporal goods are to be despised as hindering us from loving and serving God, and on the same score they are not to be feared; wherefore it is written (Sirach 34:16): "He that feareth the Lord shall tremble at nothing." But temporal goods are not to be despised, in so far as they are helping us instrumentally to attain those things that pertain to Divine fear and love.
Source: Summa Theologica (New Advent)