16 But wherefore so greatly do you fear this? Because it is said, “Thereby have fallen all that work iniquity”; so that they have come into that abyss of which it is said, “Your judgments are like the great abyss:” so that they have come even to that deep wherein sinners who despise have fallen. “Have fallen.” Whereby did they first fall? By the foot of pride. Hear the foot of pride. “When they knew God, they glorified Him not as God.” Therefore came against them the foot of pride, whereby they came into the depth.
“God gave them over to their own hearts' lusts, to do those things which are not convenient.” The root of sin, and the head of sin feared he who said, “Let not the foot of pride come against me.” Wherefore said he, “the foot”? Because by walking proudly man deserted God, and departed from Him. His foot, called he his affection. “Let not the foot of pride come against me: let not the hand of the wicked remove me:” that is, let not the works of the wicked remove me from You, that I should wish to imitate them.
But wherefore said he this against pride, “Thereby have fallen all that work iniquity”? Because those who now are ungodly, have fallen by pride. Therefore when the Lord would caution His Church, He said, “It shall watch your head, and you shall watch his heel.” The serpent watches when the foot of pride may come against you, when you may fall, that he may cast you down. But watch thou his head: the beginning of all sin is pride. “Thereby have fallen all that work iniquity: they are driven out, and are not able to stand.”
He first, who in the Truth stood not, then, through him, they whom God sent out of Paradise. Whence he, the humble, who said that he was not worthy to unloose His shoe's latchet, is not driven out, but stands and hears Him, and rejoices greatly because of the Bridegroom's voice; not because of his own, lest the foot of pride come against him, and he be driven out, and be not able to stand....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)