Here’s a good explanation of the difference between justification and sanctification. It’s found in volume four of Geerhardus Vos’ Reformed Dogmatics.
A) Sanctification serves to take away the pollution of sin and to replace it with what is good as an inherent disposition; it restores in us conformity to the image of God. Justification serves to take away the guilt of sin and to replace it with the right to eternal life in relationship with God.
The usual description (sanctification takes away the pollution of sin, justification the guilt of sin) is not incorrect but still incomplete, because only half of the matter is taken into consideration. In both, in sanctification and justification, there is a second half – namely, what is positive. To take away evil does not yet say what good is given in its place. Likewise, to abolish guilt does not yet mean to grant full rights.
B) Sanctification takes place within us, justification outside relative to us. The former is an action that changes one’s being; it must therefore intervene within our being and bring about a subjective change. The latter is a declaration that changes one’s status, a judicial word, and therefore does not affect our being but our standing; it does not take place within us but before God in His tribunal.
C) Sanctification, which begins with regeneration, has to do in the first place with the unconscious part of man. It changes, though not only but still first of all, his heart, his disposition, and from there moves to his thoughts, words, and work. Justification, which takes place in the tribunal of God, is intended to address the consciousness of man. Take note! We are not saying that justification takes place in our consciousness, although that may be said of justification in the widest sense. We are only saying that justification in the tribunal of God is intended to be perceived by the consciousness of man. That already follows from its character as a declaration. An act without a word can be applicable to unconscious life, but an act in a word, an existing verdict, must find its significance in the consciousness. Justification is thus intended to be introduced into the consciousness of man, to be brought to understanding in him.
D) Sanctification is a slow process that is never entirely completed in this life; justification, in contrast, occurs once and is complete. The former can regress; the latter cannot. Once declared, justification remains secure forever.
E) Sanctification and justification differ in their cause. The meriting cause is the same for both, namely, the merits of Christ. But the difference is to be found in the effecting cause. While God the Father as judge declares the verdict of justification in the divine tribunal, it is God the Holy Spirit, as our sanctifier, who works subjective holiness in us.
Vos, Geerhardus. Reformed Dogmatics. Edited & translated by Richard B. Gaffin Jr., vol. 4, Lexham Press, 2012–2016, pp. 138–40.
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