The Westminster Shorter Catechism
Q35 – What is sanctification?
A35 – Sanctification is the work of God’ s free grace, (2 Thess. 2:13) whereby we are renewed in the whole man after the image of God, (Eph. 4:23–24) and are enabled more and more to die unto sin, and live unto righteousness. (Rom. 6:4,6, Rom. 8:1)