Sanctification is the ongoing work of God's grace by which believers, having been united to Christ, are progressively renewed in their entire nature to die unto sin and live unto righteousness, bearing the fruit of their union with Him.
Sanctification is not merely our own effort, but a work of God's free grace. As stated in the 1689 LBCF Ch.13 §1, those united to Christ are "really and personally" sanctified through the virtue of His death and resurrection, by the indwelling of His Word and Spirit. This mirrors the teaching found in the Westminster Shorter Catechism Q.35, which defines it as the process whereby we are enabled "more and more to die unto sin, and live unto righteousness." It is a life-long transformation rooted in what Christ has already accomplished for us.
It is pastorally vital to distinguish between these two benefits of the gospel. While they are inseparably joined, the Westminster Larger Catechism Q.77 notes that in justification, God imputes Christ’s righteousness to us, pardoning our sin. In sanctification, His Spirit infuseth grace, subduing sin. Justification is a perfect status before God, whereas sanctification in this life is not yet perfect in any, but is a process of "growing up to perfection." We do not obey to become accepted; we obey because we are already accepted in the Beloved.
Ultimately, our sanctification is the fruit of Christ’s finished work. As John Owen explains in The Death of Death in the Death of Christ, the blood of Christ is the meritorious cause that secures our holiness. We are crucified with Him so that "the body of sin might be destroyed." Because Christ was raised from the dead, we are quickened to live unto God. Faith, which is the instrument of our sanctification, is itself a gift procured by His death, as seen in the believer's life today.