John 17:17
“Sanctify them through Your truth.” “Make them holy by the gift of the Spirit, and of right doctrines.” As when He says, “You are clean through the word which I spoke unto you”, so now He says the same thing, “Instruct them, teach them the truth.” “And yet He says that the Spirit does this. How then does He now ask it from the Father?” That you may again learn their equality of Honor. For right doctrines asserted concerning God sanctify the soul. And if He says that they are sanctified by the word, marvel not. And to show that He speaks of doctrines, He adds,
“Your 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 signifies nothing typical or bodily. As also Paul says concerning the Church, that He has 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.” And this is made plain from what follows. For, He says,
Source: Homilies on the Gospel of John (New Advent)