Be of sin the double cure, Save from wrath and make me pure.
-Augustus Topladay, Rock of Ages
The Baptist Faith & Message (2000) notes that,
“Sanctification is the experience, beginning in regeneration, by which the believer is set apart to God’s purposes, and is enabled to progress toward moral and spiritual maturity through the presence and power of the Holy Spirit dwelling in him. Growth in grace should continue throughout the regenerate person’s life.”
The Abstract of Principles (1858) further describes sanctification in the following manner:
“Those who have been regenerated are also sanctified, by God’s word and Spirit dwelling in them. This sanctification is progressive through the supply of Divine strength, which all saints seek to obtain, pressing after a heavenly life in cordial obedience to all Christ’s commands.”
Let’s explore that which is defined and described above. We will attempt to better understand this process by which God’s people become sanctified, or set apart, or become more holy, more like Christ. In contrast to justification, whereby one is considered righteous before God, sanctification is that lifelong process wherein one becomes more righteous and holy before God as one becomes more like Christ. In short, we could say that sanctification is God’s inevitable plan for all believers whereby He saves them from the power of sin in their lives (1 Thes 4:3; 5:23-24; 2 Thes 2:13). In sanctification, God sets believers apart unto holiness as they become more like Christ.
The nature of this process is an interesting one, for it is the work of both God and us. God commands us to work out our salvation (sanctification) with fear and trembling (Phil 2:12) and we recognize our responsibility to be more holy and less like the world (1 Pet 1:13-16). However, this is not something we do in the flesh or in our own power or strength, but in the same fashion as how we were justified (i.e., by grace through faith – Gal 3:3).
“God provided that certainty and confirms the believer’s hope, since sanctification from its beginning in regeneration to its completion in glorification is ultimately God’s work, which believers appropriate by faith (cf. Phil. 1:6).” — Bible Knowledge Commentary on Romans 8:28-39
This is a process where we work hard, knowing that He works in us and that He gets the glory for the success (1 Cor 15:10). It’s not easy, but easy doesn’t enter into grown-up life.
In sanctification we change masters for it is the process whereby slaves to sin become slaves to righteousness (cf. Romans 6-8). This is a progressive realization in the real world as we don’t become righteous overnight in the experiential sense. In the Christian life we cease our sinful ways and start doing righteous deeds for His glory (Matt 5:16).
John Owen notes that we are to be about “clearing our forests of the sinful trees and planting new and good trees in their places.” Killing off the old and sinful trees is called mortification, whereby we kill the flesh. The planting of the new and righteous is called vivification, whereby we begin and practice new and godly habits, showing forth the fruit of the Spirit (Gal 5:22-23).
Again, justification is by grace alone through faith alone in Christ alone. One is justified prior to works, but the works come after and as a result of faith (Eph 2:8-10), a changed life (2 Cor 5:17), and a new heart (Ezek 36:26-27). As a result, all of God’s children will carry on in the faith, growing in grace and will never ultimately fall away (John 10:28-29). Good works and a changed life are one of the means whereby one can know of his/her salvation (i.e., legitimate justification) (2 Cor 13:5). Others would be the internal witness of the Holy Spirit (Rom 8:16) and the evidence of God’s discipline (Heb 12:5-8).
Although sanctification is a process that has external manifestation, it is primarily an internal process. That is, we are essentially changed from the inside out. Paul notes that we are transformed by the “renewing” of our “mind” (Rom 12:2). As such, our internal motives and external actions come into alignment as we see our actions AND our motives purified. This renewing primarily occurs through teaching, preaching, prayer, Bible study and meditation.
One must never forget that our sanctification has proponents (ourselves, God, and the church – Heb 10:24-25) AND opponents (the world, the flesh, and the devil). That means that the life of holiness is a life of battle, a spiritual warfare (Eph 6:10-18). God equips believers with certain spiritual gifts (Rom 12:3-8; Eph 4:11-16) so that they can encourage and move one another on toward godliness, especially as they put on the full armor of God (Eph 6:10-18).
The life of holiness is the life of faith in which the believer, with a deepening knowledge of his own sin and helplessness apart from Christ, increasingly casts himself upon the Lord, and seeks the power of the Spirit and the wisdom and comfort of the Bible to battle against the world, the flesh, and the devil. – Edmund P. Clowney, The Church
Remember that we will fight against sin until the day of glorification, never giving up although it will get nasty.
“Let no man think to kill sin with few, easy, or gentle strokes. He who hath once smitten a serpent, if he follow not on his blow until it be slain, may repent that ever he began the quarrel. And so he who undertakes to deal with sin, and pursues it not constantly to the death.” – John Owen
However, we will all face various temptations along the way. Thus, accountability can be crucial in fighting off sin and temptation. Owen notes that in order for fallen humanity to experience legitimate temptation two things must be present: (1) Desire and (2) Opportunity (cf. James 1:13-15). In the absence of either, temptation is avoided. Yet, we can be encouraged in the midst of temptation that God is always with us and will ensure that we are not tempted beyond what we can bear (1 Cor 10:13). So, we continue to live the Christian life of sanctification by grace through faith, in humble reliance on Him as we strive to bring it about.
We have no power from Christ unless we live in a persuasion that we have none of our own. Our whole spiritual life is a life of faith; and that is a life of dependence on Christ for what we have not of ourselves. – John Owen
We live the Christian life (sanctification) the same way we began it (justification), by grace through faith. In that process we become someone else and we no longer do some of things we used to do. We mature in the process of growing up, no longer babes in Christ. There’s life change and your light shines so that people will see the difference Christ makes in a life and God will be glorified (Matthew 5:16).
 One needs to make the distinction between the “perseverance of the saints” and “eternal security”. The former assumes the latter, but is much more. The Bible does not merely teach “once saved, always saved,” although it does indeed teach that, but it makes the assertion that a genuine conversion will manifest itself in a progressive change from which a believer can and never will fall away, since God ensures success (Phil 1:6). So, the Bible offers no assurance or “security” to one without sanctification, for true faith will produce. To quote again from the Abstract of Principles: “It [Faith] is wrought in the heart by the Holy Spirit, and is accompanied by all other saving graces, and leads to a life of holiness.”
 The ordinances, or sacraments, of the church (i.e., baptism and the Lord’s Supper) are given for our sanctification as well the church in general. Per the Abstract of Principles, “The Lord Jesus is the Head of the Church, which is composed of all his true disciples, and in Him is invested supremely all power for its government. According to his commandment, Christians are to associate themselves into particular societies or churches; and to each of these churches he hath given needful authority for administering that order, discipline and worship which he hath appointed.” Thus, anyone not “in church” goes into battle for sanctification not firing on all cylinders.
 For example, if I am lusting after chocolate cake (desire) but there is none around (opportunity), temptation is not really there. Vice versa, if there is a plate full of green beans (opportunity), but I would rather stick needles in my eyes than eat them (desire), then I’m not really tempted to eat them.