Sanctify them through thy truth. — Better, in Thy truth. Truth was the sphere in which their sanctification was to take place. They had through Christ received the Father’s word, which was truth, and had passed into a new region of life, separate from the world (John 17:6-8; John 17:14-16). He has prayed that the Father would preserve them in this, and now He prays further that the Father would in this new region of life set them apart for the work to which He had sent them (John 17:18).
The idea at the root of the word rendered “sanctify,” is not holiness, but separation. It is opposed not to what is impure, but to what is common, and is constantly used in the Greek of the Old Testament for the consecration of persons and things to the service of God. Hence our Lord can use it of Himself in John 10:36, and in this context (John 17:19; these are the only places where it occurs in St. John’s writings). He was Himself “set apart and sent into the world.” He has to send them into the world in the same way (John 17:18, and John 10:36), and prays that they may be in the same way consecrated for their work.
Thy word is truth. — There is a strong emphasis in the pronoun “Thy word is truth.” This word they had kept (John 17:6-8). It had become the region of their life. They are to be the channels through which it is to pass to others (John 17:20). They are already in the higher sphere of truth, in which their entire consecration is to take place, when the gifts of the Holy Spirit shall descend upon them.