Q&A: Sanctified by God's truth or by God's Spirit?

Question

Jesus prayed that his people would be sanctified by the Word (John 17:17), but Peter says we are sanctified by God's Spirit (1 Pet. 1:2). Which is it?

Answer

John 17:17 Sanctify them by the truth; your word is truth.
1 Peter 1:2 ...who have been chosen according to the foreknowledge of God the Father, through the sanctifying work of the Spirit, to be obedient to Jesus Christ and sprinkled with his blood.

The answer is both because the Holy Spirit is the Spirit of truth.

John 15:26 When the Advocate comes, whom I will send to you from the Father - the Spirit of truth who goes out from the Father - he will testify about me.
John 16:13 But when he, the Spirit of truth, comes, he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come.

God is truth (Deut. 32:34; Ps. 19:9; 33:4; 57:10; John 1:1; 8:31-32; 17:2-3) and the Holy Spirit is the third person of the Trinity and thus by necessity is truth as well. He eternally proceeded from the Father and the Son, who are truth (John 4:24; WCF 2; WLC Q. 8-11). Another way to understand this is to say we are sanctified through God's truth - the Word (instrumental cause), which is from God's Spirit (efficient cause) - 2 Thess. 2:10-13.