4 Thereafter He adds, “To this end was I born, and for this cause came I into the world, that I should bear witness unto the truth.”** Whence it is evident that He here referred to His own temporal nativity, when by becoming incarnate He came into the world, and not to that which had no beginning, whereby He was God through whom the Father created the world. For this, then, that is, on this account, He declared that He was born, and to this end He came into the world, to wit, by being born of the Virgin, that He might bear witness unto the truth. But because all men have not faith, He still further said, “Every one that is of the truth hears my voice.” He hears, that is to say, with the ears of the inward man, or, in other words, He obeys my voice, which is equivalent to saying, He believes me. When Christ, therefore, bears witness unto the truth, He bears witness, of course, unto Himself; for from His own lips are the words, “I am the truth;” as He said also in another place, “I bear witness of myself.” But when He said, “Every one that is of the truth hears my voice,” He commends the grace whereby He calls according to His own purpose. Of which purpose the apostle says, “We know that all things work together for good to them that love God, to those who are called according to the purpose of God,” to wit, the purpose of Him that calls, not of those who are called; which is put still more clearly in another place in this way, “Labor together in the gospel according to the power of God, who saves us and calls us with His holy calling, not according to our works, but according to His own purpose and grace.” For if our thoughts turn to the nature wherein we have been created, inasmuch as we were all created by the Truth, who is there that is not of the truth? But it is not all to whom it is given of the truth to hear, that is, to obey the truth, and to believe in the truth; while in no case certainly is there any preceding of merit, lest grace should cease to be grace. For had He said, Every one that hears my voice is of the truth, then it would be supposed that he was declared to be of the truth because he conforms to the truth; it is not this, however, that He says, but, “Every one that is of the truth hears my voice.” And in this way he is not of the truth simply because he hears His voice; but only on this account he hears, because he is of the truth, that is, because this is a gift bestowed on him of the truth. And what else is this, but that by Christ's gracious bestowal he believes in Christ?
5. “Pilate said unto Him, What is truth?” Nor did he wait to hear the answer; but “when he had said this, he went out again unto the Jews, and said unto them, I find in him no fault. But you have a custom that I should release unto you one at the passover: will you therefore that I release unto you the King of the Jews?” I believe when Pilate said, “What is truth?” there immediately occurred to his mind the custom of the Jews, according to which he was wont to release unto them one at the passover; and therefore he did not wait to hear Jesus' answer to his question, What is truth? To avoid delay on recollecting the custom whereby He might be released unto them during the passover— a thing which it is clear he greatly desired. It could not, however, be torn from his heart that Jesus was the King of the Jews, but was fixed there, as in the superscription, by the truth itself, whereof he had just inquired what it was. “But on hearing this, they all cried again, saying, Not this man, but Barabbas. Now Barabbas was a robber.” We blame you not, O Jews, for liberating the guilty during the passover, but for slaying the innocent; and yet unless that were done, the true passover would not take place. But a shadow of the truth was retained by the erring Jews, and by a marvellous dispensation of divine wisdom the truth of that same shadow was fulfilled by deluded men; because in order that the true passover might be kept, Christ was led as a sheep to the sacrificial slaughter. Hence there follows the account of the injurious treatment received by Christ at the hands of Pilate and his cohort; but this must be taken up in another discourse.
Source: Tractates on the Gospel of John (New Advent)