Adam Clarke on John 17:17

Sanctify them - Ἁγιασον, from α, negative, and γη, the earth. This word has two meanings:

1. It signifies to consecrate, to separate from earth and common use, and to devote or dedicate to God and his service.

2. It signifies to make holy or pure. The prayer of Christ may be understood in both these senses. He prayed -

1. That they might be fully consecrated to the work of the ministry, and separated from all worldly concerns.

2. That they might be holy, and patterns of all holiness to those to whom they announced the salvation of God.

A minister who engages himself in worldly concerns is a reproach to the Gospel; and he who is not saved from his own sins can with a bad grace recommend salvation to others.

Through thy truth - It is not only according to the truth of God that ministers are to be set apart to the sacred work; but it is from that truth, and according to it, that they must preach to others. That doctrine which is not drawn from the truth of God can never save souls. God blesses no word but his own; because none is truth, without mixture of error, but that which has proceeded from himself. Our Lord still acts here in reference to the conduct of the high priest, to whom it belonged to sanctify the priests, the sons of Aaron: see on Joh 17:1 (note).