Justification and sanctification are two concepts that are related and inseparable; however, they represent two different or distinct notions: Justification and sanctification are indeed two benefits given by God through Jesus Christ.
In virtue of the assertion above, one may say that justification and sanctification are God’s gifts because “Christ was given to us by God’s generosity.” (Calvin, Institutes, III.11.1.) The problem arises when one merges both terms in one and uses it, as they were interchangeable. This error should be avoided in order to understand better and coherently the doctrine of salvation and union with Christ in light of the Scriptures.
For Calvin, justification is “the acceptance with which God received us into his favor as righteous men. And we say that it consists in the remission of sins and the imputation of Christ’s righteousness.”(Inst. III.11.2.) That is, once sin is forgiven due to Christ’s sacrifice, God imparts Christ’s righteousness on the believer. In other words, justification deals with God’s judgment towards the human being. This means that justification is a legal procedure that God alone started since He was the person aggravated by the human sin. Hence, justification is made for our benefit and that God does not justify believers because they are innocent but by the imputation of Christ’s righteousness. It is Christ who was innocent, who did not commit any sin, and who was free of any guilt, not the human being. Consequently, because of Christ’s sacrificial work on the cross, believers are, however, the recipients of one grace here: the reconciliation to God trough Christ’s blamelessness.
Sanctification has as objective the internal transformation and growing in God’s grace. It is done through the power of the Holy Spirit and is interchangeable with repentance, as Calvin states: “Repentance can thus well be defined: it is the true turning of our life to God, a turning that arises from a pure and earnest fear of him; and it consists in the mortification of our flesh and of the old man, and in the vivification of the Spirit.” (Inst, III.3.5.) Sanctification then is a long process that it will last the life of the believer. The Holy Spirit is always working on believers internally to make us in Christ’s image, and it is in this process, they are being sanctified. This reasoning has support in I Thessalonians 5:23 when Paul clearly states, “Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ.”
In other words, sanctification is an activity of a different kind of justification. While the first one is an ongoing process in the life of a believer, the second one is a legal procedure. Sanctification is then about being transformed (Heb. 10:14) and not about being saved or not. In the process of transformation, believers receive here another God’s gift by His grace the right to have the Holy Spirit working in our lives so that we may be transformed in the likeness of Christ: the regeneration of the believer. It is God’s will that the believer may experience this regeneration in the Spirit, as the Scriptures affirm: “For this is the will of God, your sanctification.” (cf. 1 The. 4:3) As a result, regeneration and reconciliation are evidently closely related. Besides, the author of Hebrews tells us that “Jesus also suffered outside the gate in order to sanctify the people through his own blood.” (Heb. 13:12)) As seen, Christ’s sacrifice made possible the sanctification of the people, but it does not sanctify them. So that God’s people be sanctified, the active work of the Holy Spirit is needed in the believer’s life since they cannot be sanctified by themselves.