The practice of sanctification
Sanctification is to be done by the power of God and by the power of the Holy Spirit (Gal. 3:3, 5:16). While it is we who need to be striving towards holiness, we are not to be striving on our own strength but in dependence upon the Holy Spirit who empowers us.
The practice of sanctification comes in two parts: Mortification, putting to death the Old Man (2 Cor. 5:17), or putting to death the sinful desires, and Vivification, the bringing to life the New Man, or endeavoring to live in new obedience. Mortification is to be done by repentance of sins committed, while vivification is to be done by having one's mind fixated on not doing the previous sin but instead doing what God commands, a turn-around from the previous sins. This is succinctly presented as follows:
Q87: What is repentance unto life?
A: Repentance unto life is a saving grace, whereby a sinner, out of a true sense of his sin, and apprehension of the mercy of God in Christ, does, with grief and hatred of his sin, turn from it unto God, with full purpose of, and endeavor after, new obedience (Westminster Shorter Catechism 87)
There are two dangers when it comes to the practice of sanctification. The first danger is to neglect the practice of sanctification and think that sanctification is automatic. It is true that sanctification flows from the new life in Christ, but we are called to strive to be holy, which requires making conscious effort towards godliness. The second danger is to over-emphasize sanctification such that the Christian life become fixated on holiness. Yes, holiness is important, but Christians still sin while on this world.
The first danger is a real danger. Nevertheless, the tendency is towards the second danger, because over-emphasizing sanctification lets people move towards works-righteousness while seemingly they preserve the true Gospel. When it comes to practice, the tendency for the flesh is to make personal holiness into a standard for evaluating whether a person is or is not a "real Christian," whether as applied to himself or, more commonly, to others. All of such is legalism. Yes, all believers are to grow in holiness, but sometimes sanctification is gradual and people change slowly. We must also understand that it is the natural temperament of some people to be more moral in certain areas. Therefore, a person, Person A, naturally deficient in temperament may look more ungodly than Person B who has a better natural endowment, who could be a new Christian but might not even be a believer. Since natural temperament varies a lot, we should refrain from passing hasty judgments about how much or how little sanctified a person might be, for how would you know his background?
The practice of sanctification is personal, for one's own striving towards holiness. It is not meant to be used to judge and condemn fellow believers for failing to live up to whatever standard, biblical though they might be, unless they are outright, obvious and serious sins. We are to exhort each other towards greater holiness, but that is different from condemning others for failing to live up to your standards no matter how godly they may personally be. After all, "it is before his own master that he stands or falls" (Rom. 14:4). If one is to desire greater holiness in the people of God, pray for the continual proclamation of the true Gospel, the continual calling to repentance and faith, and the continual exhortation to holiness, then let the Spirit of God do His work in the hearts of men.