Objection 1. It seems that the precepts of fortitude are not suitably given in the Divine Law. For the New Law is more perfect than the Old Law. Yet the Old Law contains precepts of fortitude (Deuteronomy 20). Therefore precepts of fortitude should have been given in the New Law also.
Objection 2. Further, affirmative precepts are of greater import than negative precepts, since the affirmative include the negative, but not vice versa. Therefore it is unsuitable for the Divine Law to contain none but negative precepts in prohibition of fear.
Objection 3. Further, fortitude is one of the principal virtues, as stated above (123, 2; I-II, 61, 2). Now the precepts are directed to the virtues as to their end: wherefore they should be proportionate to them. Therefore the precepts of fortitude should have been placed among the precepts of the decalogue, which are the chief precepts of the Law.
On the contrary, stands Holy Writ which contains these precepts.
I answer that, Precepts of law are directed to the end intended by the lawgiver. Wherefore precepts of law must needs be framed in various ways according to the various ends intended by lawgivers, so that even in human affairs there are laws of democracies, others of kingdoms, and others again of tyrannical governments. Now the end of the Divine Law is that man may adhere to God: wherefore the Divine Law contains precepts both of fortitude and of the other virtues, with a view to directing the mind to God. For this reason it is written (Deuteronomy 20:3-4): "Fear ye them not: because the Lord your God is in the midst of you, and will fight for you against your enemies."
As to human laws, they are directed to certain earthly goods, and among them we find precepts of fortitude according to the requirements of those goods.
Reply to Objection 1. The Old Testament contained temporal promises, while the promises of the New Testament are spiritual and eternal, according to Augustine (Contra Faust. iv). Hence in the Old Law there was need for the people to be taught how to fight, even in a bodily contest, in order to obtain an earthly possession. But in the New Testament men were to be taught how to come to the possession of eternal life by fighting spiritually, according to Matthew 11:12, "The kingdom of heaven suffereth violence, and the violent bear it away." Hence Peter commands (1 Peter 5:8-9): "Your adversary the devil, as a roaring lion, goeth about, seeking whom he may devour: whom resist ye, strong in faith," as also James 4:7: "Resist the devil, and he will fly from you." Since, however, men while tending to spiritual goods may be withdrawn from them by corporal dangers, precepts of fortitude had to be given even in the New Law, that they might bravely endure temporal evils, according to Matthew 10:28, "Fear ye not them that kill the body."
Reply to Objection 2. The law gives general directions in its precepts. But the things that have to be done in cases of danger are not, like the things to be avoided, reducible to some common thing. Hence the precepts of fortitude are negative rather than affirmative.
Reply to Objection 3. As stated above (Question 122, Article 1), the precepts of the decalogue are placed in the Law, as first principles, which need to be known to all from the outset. Wherefore the precepts of the decalogue had to be chiefly about those acts of justice in which the notion of duty is manifest, and not about acts of fortitude, because it is not so evident that it is a duty for a person not to fear dangers of death.
Source: Summa Theologica (New Advent)