What’s the difference between justification and sanctification?

Both justification and sanctification are different aspects of a believer’s eternal salvation in Christ (Heb. 2:3; 5:9), and both are graces of the gospel and lie at its very heart.

Question 33 of the Westminster Shorter Catechism defines justification as "an act of God’s free grace, wherein he pardoneth all our sins, and accepteth us as righteous in his sight, only for the righteousness of Christ imputed to us, and received by faith alone." So, justification is the deliverance from the penalty of sin that was accomplished by Christ for his people at the Cross of Calvary. As the apostle Paul wrote, "Therefore, as one trespass [Adam’s sin] led to condemnation for all men, so one act of righteousness [by Christ] leads to justification and life for all men" (Rom. 5:18).

Justification is an act of the free grace of God alone toward sinners who are individually guilty and deserving of God’s eternal wrath (Rom. 3:25; 6:23). It means that every believer is completely free from the condemnation and the wrath of God (Rom. 8:1, 33-34; Col. 2:13-14). Fully and finally free! It is a divine act (cf. Rom. 5:1). It happens only one time to a believer and, unlike sanctification, it is not a continuing process. It is a completed work in all believers; it can neither be reversed nor repeated (cf. Heb. 10:2, 10, 12). So, justification is a once-and-for-all act. Christians may not be equally mature in Christ, but they are each equally justified (Acts 13:39).

Justification does not describe the way that God inwardly renews and changes a person. Rather, it is a forensic act describing the legal standing of a justified person before a holy God (Rom. 5:1). In essence, it is a courtroom declaration of being found not guilty based on Christ’s righteousness alone and not on a person's works. A legal declaration of one’s standing before God does not mean the believer is inwardly sinless or pure. It speaks not of the purification of one’s nature, but of one’s righteous position before God.

God justifies the ungodly, not the godly (Rom. 5:5). How does he do this? He credits the righteousness of Christ’s perfect obedience and full satisfaction for sin to the believer’s account. This free act of God’s grace proceeds on the grounds of the imputed righteousness of Christ (cf. Rom. 5:19). The believer is saved not only by Christ’s death but also Christ's perfect sinless life. The basis for this divine declaration is the doing and dying of Christ alone. Christ is the righteousness of all those who are justified (1 Cor. 1:30).

Justification is received by faith without works (Rom. 3:20-22; 4:1-8, 24; 5:1; Gal. 3:5-12). It is not deserved by the sinner, nor can it be earned or merited. A believer’s faith adds absolutely nothing to what Christ has done for them in justification; it only receives the righteousness of Christ offered in the gospel (Rom. 4:4-5). Essentially, the sinner comes empty-handed to the market of free grace for remission of sins and God’s favor (cf. Isa. 55:1; Matt. 10:8).

By contrast, sanctification is not the act of God declaring a person righteous. Rather, as the Westminster Shorter Catechism Q. 35 states, it is "the work of God's free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness." It is the continuous process of believers becoming more Christlike (cf. Rom. 8:29). It is the continuous, progressive renewing and transforming of our whole person – our affections, behaviors, minds, thoughts, bodies and wills.

Sanctification is both God’s doing and ours. As to God’s part, it is the Holy Spirit working in his beloved both to will and do of his good pleasure (Phil. 2:13). It includes the saint’s continuous deliverance from the power of sin — "the desires of the flesh and the desires of the eyes and pride of life (1 John 2:16). It also incorporates his working in us to live unto righteousness (Phil. 2:12; cf. Gal. 5:22-23), "being confident of this, that He who began a good work in [us] will continue to perfect it until the day of Christ Jesus" (Phil. 1:6, NASB).

As to our part, we are to count ourselves dead to sin, but alive to God and his righteousness (Rom. 6:11). While the power of sin in us is broken, we still have to take active steps against it. Though sin's dungeon doors have been opened, we still have to take steps necessary to exit it and never look back. We need to not let sin reign in our bodies. We need not offer any part of ourselves to sin (Rom. 6:12-13). Instead, we offer ourselves as willing bond-slaves to righteous (Rom. 6:22). So, our part includes the continuous process of counting ourselves dead to sin but alive to God in Christ Jesus (Rom. 6:11).

The Holy Spirit’s goal in the believer’s sanctification is that we will be found in Christ’s image — "so that when he appears we may be confident and unashamed before him at his coming" (1 John 2:28). As Paul says in Romans 8:29-30, "For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified."

In summary, justification happens after regeneration and conversion. Sanctification begins after justification and is ongoing up to the time of glorification. Justification is a declaration of one’s righteousness; sanctification is the process of making one more righteous. Justification is a once-and-for-all divine act; sanctification is a continual process. Justification addressees the guilt of our sin; sanctification addresses the dominion and corruption of our sin.

Both these graces concern different aspects of the believer’s faith in Jesus Christ. In justification, the believer’s faith results in his freedom from sin and his acceptance, being declared fully righteous in God’s sight. In sanctification, this very same faith is enabled to actively and enthusiastically take up all the commands of Christ and live them for God’s glory alone.