5 For He showed that He would break off these branches, and ingraft this wild olive, when moved by the faith of the centurion, who said to Him, “I am not worthy that you should come under my roof; but only speak the word, and my child shall be healed: for I also am a man put under authority, having soldiers under me; and I say to one, Go, and he goes; and to another, Come, and he comes; and to my servant, Do this, and he does it. Jesus turned to those who followed Him, and said, Verily I say unto you, I have not found so great faith in Israel.”
Why not found so great faith in Israel? “Because a prophet has no honor in his own country.” Could not the Lord have said to that centurion, what He said to this ruler, “Go, your child lives?” See the distinction: this ruler desired the Lord to come down to his house; that centurion declared himself to be unworthy. To the one it was said, “I will come and heal him;” to the other, “Go, your son lives.” To the one He promised His presence; the other He healed by His word. The ruler sought His presence by force; the centurion declared himself unworthy of His presence.
Here is a ceding to loftiness; there, a conceding to humility. As if He said to the ruler, “Go, your son lives;” do not weary me. “Unless you see signs and wonders, you believe not;” you desire my presence in your house, I am able to command by a word; do not wish to believe in virtue of signs: the centurion, an alien, believed me able to work by a word, and believed before I did it; you, “except ye see signs and wonders, believe not.” Therefore, if it be so, let them be broken off as proud branches, and let the humble wild olive be grafted; nevertheless let the root remain, while those are cut off and these received in their place.
Where does the root remain? In the patriarchs. For the people Israel is Christ's own country, since it is of them that He came according to the flesh; but the root of this tree is Abraham, Isaac, and Jacob, the holy patriarchs. And where are they? In rest with God, in great honor; so that it was into Abraham's bosom that the poor man, on being promoted, was raised after his departure from the body, and in Abraham's bosom was he seen from afar off by the proud rich man. Wherefore the root remains, the root is praised; but the proud branches deserved to be cut off, and to wither away; and by their cutting off, the humble wild olive has found a place.
Source: Tractates on the Gospel of John (New Advent)