Westminster Larger Catechism in modern English question number seventy-five:
Q. 75. What is sanctification?
A. Sanctification is a work of God’s grace, by which those whom God has, before the foundation of the world, chosen to be holy, are, in time and through the powerful operation of His Spirit applying the death and resurrection of Christ to them, renewed in their whole person after the image of God. This is accomplished by their having the seeds of repentance unto life and all other saving graces put into their hearts and those graces so stirred up, increased, and strengthened, so that they more and more die unto sin and rise unto newness of life.