A sin of omission occurs in two ways: one, when a person fails to do good; another, by failing to endure evil and adversity. In regard to the first he says, Therefore, i.e., because chastisement yields the most peaceable fruit, then to obtain this fruit, lift your drooping hands. For since the hand is the organ of the organs, it is said to droop, when it stops performing good works; therefore, it must be lifted up by a right intention to do things pleasing to God: ‘Let us lift up our hearts with our hands to God’ (Lam 3:41); ‘The lifting up of my hands as an evening sacrifice’ (Ps. 140:2); ‘The slothful hand has wrought poverty; but the hand of the industrious riches’ (Pr. 10:4); ‘The hand of the valiant shall bear rule, but that which is slothful shall be under tribute’ (Pr. 12:24). As a sign of this, when Moses lifted up his hands, Israel conquered; but when he let them fall, Amalek overcame them (Ex. 17:11). In regard to the other sin of omission he says, strengthen your weak knees. The entire weight of the body is held up by the knees. Therefore, those who have not the courage to endure adversity bravely have weak knees. Therefore, this weakness must be put aside: ‘You have strengthened the weary hands; your words have confirmed them that were staggering, and you have strengthened the trembling knees’ (Jb. 4:3); ‘Strengthen the feeble hands and confirm the weak knees’ (Is. 35:3). Therefore, lift up the hand and knees and do not give in to idleness or hesitate because of weakness.
Commentary on the Epistle to the Hebrews, c. 12 l. 3, section 684