Sanctify them in the truth.John 17:17
Sanctification begins in regeneration. The Spirit of God implants in man that new living principle by which he becomes "a new creation"1 in Christ Jesus. This work, which begins in the new birth, is carried on in two ways-mortification, whereby the lusts of the flesh are subdued and kept under, and vivification, by which the life that God has put within us is made to be a well of water springing up unto everlasting life.
This is carried on every day in what is called perseverance, by which the Christian is preserved and continued in a gracious state and is made to abound in good works unto the praise and glory of God; and it culminates or comes to perfection in glory, when the soul, being thoroughly purged, is caught up to dwell with holy beings at the right hand of the Majesty on high.
But while the Spirit of God is thus the author of sanctification, yet there is a visible agency employed that must not be forgotten. "Sanctify them," said Jesus, "in the truth; your word is truth." The passages of Scripture that prove that the instrument of our sanctification is the Word of God are numerous. The Spirit of God brings to our minds the precepts and doctrines of truth and applies them with power. These are heard in the ear, and being received in the heart, they work in us to will and to do God's good pleasure.
The truth is the sanctifier, and if we do not hear or read the truth, we shall not grow in sanctification. We only progress in sound living as we progress in sound understanding. "Your word is a lamp to my feet and a light to my path."2 Do not say of any error, "It is a mere matter of opinion." No man indulges an error of judgment without sooner or later tolerating an error in practice. Hold fast the truth, for by doing so you shall be sanctified by the Spirit of God.
1) 2 Corinthians 5:17
2) Psalm 119:105