6 And what has he said? “And wherefore shall I fear in the evil day? The iniquity of my heel shall compass me”. He begins something obscurely. Therefore he ought the rather to fear if the iniquity of his heel shall compass him. Nay, for let not man fear, he says, who has not power to escape. For example, he who fears death, what shall he do to escape death? Let him tell me how he is to escape what Adam owes, he who is born of Adam. But let him consider that he is born of Adam, and has followed Christ, and ought to pay what Adam owes, and obtain what Christ has promised.
Therefore, he who fears death can no wise escape: but he who fears the damnation which the ungodly shall hear, “Go ye into everlasting fire,” has an escape. Let him not fear then. For why should he fear? Will the iniquity of his heel compass him? If then he avoid “the iniquity of his heel,” and walk in the ways of God, he shall not come to the evil day: the evil day, the last day, shall not be evil to him....Now while they live, let them take heed to themselves, let them put away iniquity from their heel: let them walk in that way, let them walk in the way of which He says Himself, “I am the way, the truth, and the life:” and let them not fear in the evil day, for He gives them safety who became “The Way.”
Therefore let them avoid the iniquity of their heel. With the heel a man slips. Let your Love observe. What was said by God to the Serpent? “She shall mark your head, and you shall mark her heel.” The devil marks your heel, in order that when you slip he may overthrow you. He marks your heel, do thou mark his head. What is his head? The beginning of an evil suggestion. When he begins to suggest evil thoughts, then you thrust him away before pleasure arises, and consent follows; and so shall you avoid his head, and he shall not grasp your heel.
But wherefore said He this to Eve? Because through the flesh man does slip. Our flesh is an Eve within us. “He that loves his wife,” he says, “loves himself.” What means “himself”? He continues, and says, “For no man ever yet has hated his own flesh.” Because then the devil would make us slip through the flesh, just as he made that man Adam to slip, through Eve; Eve is bidden to mark the head of the devil, because the devil marks her heel. “If then the iniquity of our heel shall compass us, why fear we in the evil day,” since being converted to Christ we are able not to do iniquity; and there will be nothing to compass us, and we shall joy and not sorrow in the last day?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)