The Protestant Reformation cleared much of the theological fog that was hanging in the church. One area where the fog was cleared is in the doctrines of justification and sanctification. Justification and sanctification had been mixed in such a way the church was teaching that a person is justified insofar as he is sanctified. The Reformers, however, rightly highlighted the biblical distinction. The Westminster Larger Catechism puts it this way in Q/A 77:
Although sanctification be inseparably joined with justification, (1 Cor. 6:11, 1 Cor. 1:30) yet they differ, in that God in justification imputeth the righteousness of Christ; (Rom. 4:6 ,8) in sanctification of his Spirit infuseth grace, and enableth to the exercise thereof; (Ezek. 36:27) in the former, sin is pardoned; (Rom. 3:24–25) in the other, it is subdued: (Rom. 6:6,14) the one doth equally free all believers from the revenging wrath of God, and that perfectly in this life, that they never fall into condemnation (Rom. 8:33–34) the other is neither equal in all, (1 John 2:12–14, Heb. 5:12–14) nor in this life perfect in any, (1 John 1:8,10) but growing up to perfection. (2 Cor. 7:1, Phil. 3:12–14),
Louis Berkhof summarizes this well in his Systematic Theology:
The following points of difference between justification and sanctification should be carefully noted:1. Justification removes the guilt of sin and restores the sinner to all the filial rights involved in his state as a child of God, including an eternal inheritance. Sanctification removes the pollution of sin and renews the sinner ever-increasingly in conformity with the image of God.2. Justification takes place outside of the sinner in the tribunal of God, and does not change his inner life, though the sentence is brought home to him subjectively. Sanctification, on the other hand, takes place in the inner life of man and gradually affects his whole being.3. Justification takes place once for all. It is not repeated, neither is it a process; it is complete at once and for all time. There is no more or less in justification; man is either fully justified, or he is not justified at all. In distinction from it sanctification is a continuous process, which is never completed in this life.4. While the meritorious cause of both lies in the merits of Christ, there is a difference in the efficient cause. Speaking economically, God the Father declares the sinner righteous, and God the Holy Spirit sanctifies him.
These are proper – and biblical – distinctions that we must maintian to uphold the gospel of grace, the perfect work of Christ, the glory of God in salvation, and the assurance of the Christian’s faith.
The above quote is found in Louis Berkhof, Systematic Theology (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1938), 513–514.