John 17:17

"Sanctify them through Thy truth." "Make them holy by the gift of the Spirit, and of right doctrines." As when He saith, "Ye are clean through the word which I spake unto you," so now He saith the same thing, "Instruct them, teach them the truth." "And yet He saith that the Spirit doth this. How then doth He now ask it from the Father?" That thou mayest again learn their equality of Honor. For right doctrines asserted concerning God sanctify the soul. And if He saith that they are sanctified by the word, marvel not. And to show that He speaketh of doctrines, He addeth, "Thy word is truth." That is, "there is no falsehood in it, and all that is said in it must needs come to pass"; and again, it signifieth nothing typical or bodily. As also Paul saith concerning the Church, that He hath sanctified it by the Word. For the Word of God is wont also to cleanse. Moreover, the, "sanctify them," seems to me to signify something else, such as this, "Set them apart for the Word and for preaching."

(Tr. cviii) Sanctify them through Thy truth: for thus were they to be kept from the evil. But it may be asked, how it was that they were not of the world, when they were not yet sanctified in the truth? Because the sanctified have still to grow in sanctity, and this by the help of God's grace. The heirs of the New Testament are sanctified in that truth, the shadows of which were the sanctification of the Old Testament; they are sanctified in Christ, Who said above, I am the way, the truth, and the life. (c. 14:6) It follows, Thy discourse is truth. The Greek is λόγος, i. e. word. The Father then sanctified them in the truth, i. e. in His Word the Only-Begotten, them, i. e. the heirs of God, and joint-heirs with Christ.

The heirs therefore of the New Testament are sanctified in that truth which was adumbrated in the purifications of the Old Testament; and when they are sanctified in the truth, they are in other words sanctified in Christ, who said in truth "I am the way, and the truth, and the life." As also when He said, "The truth shall make you free," in explanation of His words, He added soon after, "If the Son shall make you free, ye shall be free indeed;" in order to show that what He had previously called the truth, He a minute afterwards denominates the Son. And what else did He mean by the words before us, "Sanctify them in the truth," but, Sanctify them in me? Finally, He proceeds, and doing so fails not to suggest the same with increasing clearness: "Thy speech is truth." What else did He mean than "I am the truth"? For the Greek Gospel has logos, which is also the word that is found in the passage where it is said, "In the beginning was the Word, and the Word was with God, and the Word was God." And that Word at least we know to be the only begotten Son of God, which "was made flesh, and dwelt among us." Hence also there might have been put here as it actually has been put in certain copies "Thy Word is truth;" just as in some copies that other passage is written, "In the beginning was the speech." But in the Greek without any variation it is logos in both cases. The Father therefore sanctifies in the truth, that is, in His own Word, in His Only begotten, His own heirs and His (the Son's) co-heirs.