Christ came to save the world, not only by Divine power, but also through the mystery of His Incarnation. Consequently in healing the sick He frequently not only made use of His Divine power, healing by way of command, but also by applying something pertaining to His human nature. Hence on Lk. 4:40, “He, laying His hands on every one of them, healed them,” Cyril says: “Although, as God, He might, by one word, have driven out all diseases, yet He touched them, showing that His own flesh was endowed with a healing virtue.” And
on Mk. 8:23, “Spitting upon his eyes, laying His hands on him,” etc., Chrysostom [Victor of Antioch] says: "He spat and laid His hands upon the blind man, wishing to show that His Divine word, accompanied by His operation, works wonders: for the hand signifies operation; the spittle signifies the word which proceeds from the mouth." Again, on John 9:6, “He made clay of the spittle, and spread the clay upon the eyes of the blind man,” Augustine says: “Of His spittle He made clay–because ‘the Word was made flesh.’” Or, again, as Chrysostom says, to signify that it was He who made man of “the slime of the earth.”
It is furthermore to be observed concerning Christ’s miracles that generally what He did was most perfect. Hence on John 2:10, “Every man at first setteth forth good wine,” Chrysostom says: “Christ’s miracles are such as to far surpass the works of nature in splendor and usefulness.” Likewise in an instant He conferred perfect health on the sick. Hence on Mt. 8:15, “She arose and ministered to them,” Jerome says: “Health restored by our Lord returns wholly and instantly.”
There was, however, special reason for the contrary happening in the case of the man born blind, and this was
his want of faith, as Chrysostom [Victor of Antioch] says. Or ***as Bede observes on Mk. 8:23: “Whom He might have healed wholly and instantly by a single word, He heals little by little, to show the extent of human blindness, which hardly, and that only by degrees, can come back to the light: and to point out that each step forward in the way of perfection is due to the help of His grace.” ***
(
Summa Theologica, III, 44, 3)