The primary meaning of sanctification is to “set apart”, to separate an object or person from its basic value. For example we might go to a library and separate out one book from all the rest on the shelf. In the Bible that "setting apart" is to make it possible that it or the person can be used in the service of God. In connection with objects-
“Thou shall anoint the altar.., and sanctify the altar ……thou shall anoint the laver . . and sanctify it “(Exodus 40:10-11).
An object of wood or metal cannot be made holy in the ordinary sense of that word, for it has no qualities of mind or character. It can, however, be solemnly set apart for divine use. So also when sanctification is referring to people, it has this primary meaning of being “set apart”.
We must carefully note, however, that for the believer sanctification has two aspects. The first is positional and absolute- an act of God with which we start our Christian life, and secondly, practical and progressive-continuing and deepening through all our pathway upon earth.
Paul wrote to the Corinthians in his first epistle as unto “them that are sanctified in Christ Jesus” (1 Cor 1:2) and again writes “but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of God” (1 Cor 6:11). These are striking sayings, for the Corinthian Christians were in many respects very blameworthy. They had not advanced far in the way of practical sanctification; yet the apostle does not hesitate to remind them, that in the name of the Lord Jesus and by God’s Spirit they had been sanctified, as truly as they had been washed and justified. They had been set apart for God. Again, in Hebrews 10:14 we read “by one offering He hath perfected for ever them that are sanctified”.
Who are these sanctified ones? Are they believers of special attainments in holiness? No, they are all Christians without distinction or class - set apart for God in virtue of the one sacrifice of our Lord Jesus Christ
Secondly, there are other scriptures where sanctification is presented as an object of attainment and desire. We read, “this is the will of God, even your sanctification” (1 Thess.4:3); “Christ loved the church and gave Himself for her; that He might sanctify and cleanse her” (Eph.5:25-26). In these scriptures though sanctification still carries it’s root meaning of setting apart, it is clearly viewed as something which is God’s intention for His people, as something which Christ is doing for His church today, as something which we are to individually seek, and which instead of being already ours by God’s gracious act, is to be ours if we respond to His divine instructions. In a word, it is sanctification of a practical and progressive sort.
But upon what does sanctification depend? In its positional aspect it is founded on the work of Christ by His one offering we are sanctified. It also stands connected with the Holy Spirit - we are “elect... through sanctification of the Spirit” (1 Peter 1:2). By the Spirit we are born again and finally in believing the truth we are sealed by that same Spirit. In the virtue of all this we are set apart for God.
Perfect sanctification is not attainable in this life. See, for example, Paul's account of himself in Rom 7:14-25; Phi 3:12-14; and 1Ti 1:15, and also the confessions of David (Psa 19:12-13; Psa 51), of Moses (Psa 90:8), of Job (Job 42:5-6), and of Daniel (Dan. 9:3-20). Believers find that their life is a constant warfare, and they need to watch while they pray. The more holy a man is, the more sensitive to every sin he becomes, and the more closely he clings to Christ.
To view sanctification as something we can produce through our own performance (that is, by law) is akin to overlooking God's grace and underestimating the provisions of the cross of Christ. "Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh? (Gal 3:3).