It is not until we understand what it means to be justified, that we are in any position to discuss sanctification, which is that life-long process through which the old habit of sin (what we call “indwelling sin”) is progressively weakened and the new nature (given us by virtue of regeneration) is progressively strengthened. Why is this the case? The same act of faith which unites us to Christ so that his merits are imputed to us and thereby provides the basis upon which God pronounces us “not guilty,” also begins the life-long process of sanctification, in which our sinful habits begin to weaken, new Godly affections begin to grow, and we begin to obey (however, feebly), not some, but all of God’s commandments. To put it yet another way, every justified sinner is also being sanctified.
In fact, the moment we place our trust in Jesus Christ, all of our sins (past, present, and future) are forgiven. Through that same act of faith which justifies us, Christ’s righteousness becomes ours so that we now rely on the obedience of Jesus Christ crucified which is ours when we believe in him. Because we are justified by the merits of Jesus Christ which we receive through the means of faith (and not through our own good works), our consciences are freed from fear, terror, and dread. Since we are not paralyzed by the fear that God will punish us when we fail, we find ourselves free to obey the law of God, not to earn greater righteousness, nor to become “holier.” Rather, we obey the law of God and do good works because we have already been reckoned as “righteous” and our eternal standing before God has already been settled by the active and passive obedience of Jesus Christ. This is what it means, in part, to be sanctified.
The biblical teaching about sanctification is quite extensive. According to Paul, this struggle with sin is the normal Christian life (Romans 7:14-25). In fact, the holiest among us may be those struggling with sin the most. The fruit of the flesh (as depicted by Paul in Galatians 5:19-21) gradually begin to diminish, while the fruit of the Spirit (v. 22-23) spontaneously begin to appear in our lives. It is not the struggle with sin, but apathy to the things of God and indifference to our personal sins which are the real signs of trouble.
The new man (who was dead in sin, but is now alive in Christ) is no longer a slave to sin. The old nature (the flesh) has been crucified with Christ and buried with him in baptism (cf. Romans 6:1-7). The new man (the regenerate nature) comes alive through the resurrection power of Christ and has an entirely different orientation than the flesh. The new nature believes God’s promises, it embraces Christ through faith, it hates sin, and it desires to please God. This is why everyone who is called through the preaching of the gospel and who then comes to faith in Christ (through the operations of the Holy Spirit) is not only justified through the means of faith, but also has a new set of desires and affections which reflect the new nature.
This is why sanctification is the necessary consequence of the once-and-for-all declaration that we have been justified. Whenever someone claims to be justified by grace alone, through faith alone, on account of Christ alone but then continues to live in indifference to sin without struggling against it, something is wrong. No justified sinner can remain indifferent about their conduct nor continue on in sin as they did before they were justified.
In the new birth we are made alive with Christ and the power which sin formerly held over us is broken. But indwelling sin (what our theologians call the habitus of sin) remains in us until we die. Romans 7:14-25 and Galatians 5:17 are very clear about this fact. The new nature must struggle against the three enemies of every Christian: the world (the non-Christian way of thinking and doing), the flesh (the sinful desires within us) and the devil (lies and falsehoods about God). This is why the struggle with sin is the necessary fruit of justification and the new birth. Sadly, this struggle has led many to question their relationship with God, when, in fact, the struggle with sin is the sure sign that God is at work, molding us, and conforming us into the image of his dear son.
In Romans 6:6, Paul speaks of us as people who were formerly slaves to sin. But once freed from our slavery, we struggle to stop thinking and acting as slaves, and we struggle to start living like the free men and women that we are. The struggle will not produce victory over all sin in this life as Christian perfectionists teach. But the power of sin is broken so that sanctification and transformation necessarily begin. And yet, the habit of sin (indwelling sin) will remain with us until we die or our Lord returns, whichever comes first.
This is why you cannot successfully argue, as certain Christians attempt to do, that someone can “accept Jesus as their Savior,” but not make him Lord over their lives until a later time (the so-called “Lordship controversy”). If you trust in Jesus Christ through faith, you have been crucified with Christ, buried with Christ, and are now alive with Christ. There is no such thing in the New Testament as a two-tiered Christian life, in which there are people who accept Christ as Savior but have not yet made him Lord, nor are there people who are indwelt by the Holy Spirit but not yet baptized by the Spirit, nor are there people who are saved, but not yet sanctified.
Therefore, as we die to sin and rise to newness of life on a daily basis, we will struggle with sin. But despite the difficulties which this struggle creates, our sanctification is a sure and certain sign that we are Christ’s and that he who has begun a good work in us, will indeed see it through to the day of completion (Philippians 1:6). And this is what it means to be sanctified.