2 “I pray not,” He adds, “that you should take them out of the world, but that You should keep them from the evil.” For they still accounted it necessary to be in the world, although they were no longer of it. Then He repeats the same statement: “They are not of the world, even as I am not of the world. Sanctify them in the truth.” For so are they kept from the evil, as He had previously prayed that they might be. But it may be inquired how they were no more of the world, if they were not yet sanctified in the truth; or, if they already were, why He requests that they should be so.
Is it not because even those who are sanctified still continue to make progress in the same sanctification, and grow in holiness; and do not so without the aid of God's grace, but by His sanctifying of their progress, even as He sanctified their outset? And hence the apostle likewise says: “He who has begun a good work in you, will perform it until the day of Jesus 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, you 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?
Source: Tractates on the Gospel of John (New Advent)