5 “Then said the chief priests of the Jews unto Pilate, Write not, The King of the Jews; but that he said, I am King of the Jews. Pilate answered, What I have written I have written.” Oh the ineffable power of the working of God, even in the hearts of the ignorant! Was there not some hidden voice that sounded through Pilate's inner man with a kind, if one may so say, of loud-toned silence, the words that had been prophesied so long before in the very letter of the Psalms, “Corrupt not the inscription of the title”? Here, then, you see, he corrupted it not; what he has written he has written.
But the high priests, who wished it to be corrupted, what did they say? “Write not, The King of the Jews; but that he said, I am King of the Jews.” What is it, madmen, that you say? Why do you oppose the doing of that which you are utterly unable to alter? Will it by any such means become the less true that Jesus said, “I am King of the Jews”? If that cannot be tampered with which Pilate has written, can that be tampered with which the truth has uttered? But is Christ king only of the Jews, or of the Gentiles also?
Yes, of the Gentiles also. For when He said in prophecy, “I am set king by Him upon His holy hill of Zion, declaring the decree of the Lord,” that no one might say, because of the hill of Zion, that He was set king over the Jews alone, He immediately added, “The Lord said unto me, You are my Son; this day have I begotten You. Ask of me, and I will give You the Gentiles for Your inheritance, and the uttermost parts of the earth for Your possession.” Whence He Himself, speaking now with His own lips among the Jews, said, “Other sheep I have which are not of this fold; them also I must bring, and they shall hear my voice, and there shall be one flock and one Shepherd.” Why then would we have some great mystery to be understood in this superscription, wherein it was written, “King of the Jews,” if Christ is king also of the Gentiles?
For this reason, because it was the wild olive tree that was made partaker of the fatness of the olive tree, and not the olive tree that was made partaker of the bitterness of the wild olive tree. For inasmuch as the title, “King of the Jews,” was truthfully written regarding Christ, who are they that are to be understood as the Jews but the seed of Abraham, the children of the promise, who are also the children of God? For “they,” says the apostle, “who are the children of the flesh, these are not the children of God; but the children of the promise are counted for the seed.” And the Gentiles were those to whom he said, “But if you be Christ's, then are you Abraham's seed, and heirs according to the promise.” Christ therefore is king of the Jews, but of those who are Jews by the circumcision of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God; who belong to the Jerusalem that is free, our eternal mother in heaven, the spiritual Sarah, who casts out the bond maid and her children from the house of liberty. And therefore what Pilate wrote he wrote, because what the Lord said He said.
Source: Tractates on the Gospel of John (New Advent)