Q: What is sanctification?
A: Sanctification is the work of God's free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness.

2 Thessalonians 2:13. God hath from the beginning chosen you to salvation through sanctification of the Spirit and of the truth.
Ephesians 4:23-24. And be renewed in the spirit of your mind; and that ye put on the new man, which after God is created in righteousness and true holiness.
Romans 6:4, 6, 14. Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. . . knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin. . . For sin shall not have dominion over you: for ye are not under the law, but under grace.
Romans 8:4. That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.

Westminster Project - See Francis Beattie further commentary on adoption and sanctification as the benefits of Redemption