16 Pride is the source of all diseases, because pride is the source of all sins. When a physician removes a disorder from the body, if he merely cures the malady produced by some particular cause, but not the cause itself, he seems to heal the patient for a time, but while the cause remains, the disease will repeat itself. For example, to speak of this more expressly, some humor in the body produces a scurf or sores; there follows a high fever, and not a little pain; certain remedies are applied to repress the scurf, and to allay that heat of the sore; the remedies are applied, and they do good; you see the man who was full of sores and scurf healed; but because that humor was not expelled, it returns again to ulcers.
The physician, perceiving this, purges away the humor, removes the cause, and there will be no more sores. Whence does iniquity abound? From pride. Cure pride and there will be no more iniquity. Consequently, that the cause of all diseases might be cured, namely, pride, the Son of God came down and was made low. Why are you proud, O man? God, for you, became low. You would perhaps be ashamed to imitate a lowly man; at any rate, imitate the lowly God. The Son of God came in the character of a man and was made low.
You are taught to become humble, not of a man to become a brute. He, being God, became man; do thou, O man, recognize that you are man. Your whole humility is to know yourself. Therefore because God teaches humility, He said, “I came not to do my own will, but the will of Him that sent me.” For this is the commendation of humility. Whereas pride does its own will, humility does the will of God. Therefore, “Whoso comes to me, I will not cast him out.” Why? “Because I came not to do my own will, but the will of Him that sent me.”
I came humble, I came to teach humility, I came a master of humility: he that comes to me is made one body with me; he that comes to me becomes humble; he who adheres to me will be humble, because he does not his own will, but the will of God; and therefore he shall not be cast out, for when he was proud he was cast out.
Source: Tractates on the Gospel of John (New Advent)