Objection 1. It seems that pusillanimity is not a sin. For every sin makes a man evil, just as every virtue makes a man good. But a fainthearted man is not evil, as the Philosopher says (Ethic. iv, 3). Therefore pusillanimity is not a sin.
Objection 2. Further, the Philosopher says (Ethic. iv, 3) that "a fainthearted man is especially one who is worthy of great goods, yet does not deem himself worthy of them." Now no one is worthy of great goods except the virtuous, since as the Philosopher again says (Ethic. iv, 3), "none but the virtuous are truly worthy of honor." Therefore the fainthearted are virtuous: and consequently pusillanimity is not a sin.
Objection 3. Further, "Pride is the beginning of all sin" (Sirach 10:15). But pusillanimity does not proceed from pride, since the proud man sets himself above what he is, while the fainthearted man withdraws from the things he is worthy of. Therefore pusillanimity is not a sin.
Objection 4. Further, the Philosopher says (Ethic. iv, 3) that "he who deems himself less worthy than he is, is said to be fainthearted." Now sometimes holy men deem themselves less worthy than they are; for instance, Moses and Jeremias, who were worthy of the office God chose them for, which they both humbly declined (Exodus 3:11; Jeremiah 1:6). Therefore pusillanimity is not a sin.
On the contrary, Nothing in human conduct is to be avoided save sin. Now pusillanimity is to be avoided: for it is written (Colossians 3:21): "Fathers, provoke not your children to indignation, lest they be discouraged." Therefore pusillanimity is a sin.
I answer that, Whatever is contrary to a natural inclination is a sin, because it is contrary to a law of nature. Now everything has a natural inclination to accomplish an action that is commensurate with its power: as is evident in all natural things, whether animate or inanimate. Now just as presumption makes a man exceed what is proportionate to his power, by striving to do more than he can, so pusillanimity makes a man fall short of what is proportionate to his power, by refusing to tend to that which is commensurate thereto. Wherefore as presumption is a sin, so is pusillanimity. Hence it is that the servant who buried in the earth the money he had received from his master, and did not trade with it through fainthearted fear, was punished by his master (Matthew 25; Luke 19).
Reply to Objection 1. The Philosopher calls those evil who injure their neighbor: and accordingly the fainthearted is said not to be evil, because he injures no one, save accidentally, by omitting to do what might be profitable to others. For Gregory says (Pastoral. i) that if "they who demur to do good to their neighbor in preaching be judged strictly, without doubt their guilt is proportionate to the good they might have done had they been less retiring."
Reply to Objection 2. Nothing hinders a person who has a virtuous habit from sinning venially and without losing the habit, or mortally and with loss of the habit of gratuitous virtue. Hence it is possible for a man, by reason of the virtue which he has, to be worthy of doing certain great things that are worthy of great honor, and yet through not trying to make use of his virtue, he sins sometimes venially, sometimes mortally.
Again it may be replied that the fainthearted is worthy of great things in proportion to his ability for virtue, ability which he derives either from a good natural disposition, or from science, or from external fortune, and if he fails to use those things for virtue, he becomes guilty of pusillanimity.
Reply to Objection 3. Even pusillanimity may in some way be the result of pride: when, to wit, a man clings too much to his own opinion, whereby he thinks himself incompetent for those things for which he is competent. Hence it is written (Proverbs 26:16): "The sluggard is wiser in his own conceit than seven men that speak sentences." For nothing hinders him from depreciating himself in some things, and having a high opinion of himself in others. Wherefore Gregory says (Pastoral. i) of Moses that "perchance he would have been proud, had he undertaken the leadership of a numerous people without misgiving: and again he would have been proud, had he refused to obey the command of his Creator."
Reply to Objection 4. Moses and Jeremias were worthy of the office to which they were appointed by God, but their worthiness was of Divine grace: yet they, considering the insufficiency of their own weakness, demurred; though not obstinately lest they should fall into pride.
Source: Summa Theologica (New Advent)