Many a good word is put to a wrong use, and the word sanctification is certainly one which is very often employed in a sense different from, and contrary to, its use in Scripture. So, before looking at the subject we must define the term.
In the Bible it generally means to set apart, hallow, dedicate, consecrate. It is the setting apart of things or persons for a special purpose, as opposed to the common usage. It does not necessarily imply a change in the composition of the article or the condition of the person. Sanctified wood does not become gold nor is it improved as timber.
The first occurrence of the word concerns the Sabbath which God sanctified (Gen. 2:3). It was to be a day set apart from the others in its use. It was not different in length or weather. A field or a house could be sanctified (Lev. 27:14, 19). Mount Sinai was sanctified (Ex. 19:23). The tabernacle and its furnishings were sanctified (Ex. 29:44; 40:10-11). The gold and the gifts in the temple (Matt. 23:17). Food can be sanctified (1 Tim. 4. 5). Nations were called "sanctified ones" for the fulfilling of Gods purpose in the destruction of Babylon (Isa. 13:3). An unbelieving husband or wife is sanctified (1 Cor. 7:14). The Father sanctified the Son (John 10:36), and the Son sanctified himself (John 17:19). Here it very clearly refers to setting apart for a special mission, and not to any improving or purifying of the Son of God.
With regard to the sanctification of Christians in order to clearer understanding, we will consider it under three headings: before conversion, at conversion, and after conversion.
SANCTIFICATION BEFORE CONVERSION
Please open your Bible and read 2 Thes. 2:13 and 1 Peter 1:2. In the former we read of "salvation through sanctification of the Spirit and belief of the truth." In the latter of "sanctification of the Spirit unto obedience and sprinkling of the blood of Jesus Christ." This sanctification precedes belief and obedience and is with a view to salvation and sprinkling of the blood. It means that God by His Spirit lays hold of sinners, separates them from the careless and unheeding, burdens them about eternal things, convinces them of sin and guilt and they are brought to faith in Christ. This is the common experience of all who have been saved. They were no different from others but were made to differ. Without this sanctification none can be saved. It is by the operation of the Spirit of God, not by the persuasion or earnestness of the preacher.
SANCTIFICATION AT CONVERSION
In the New Testament all believers are called saints, holy ones, or sanctified ones (2 Thes. 1:10). Even the unspiritual Corinthians were sanctified in Christ Jesus and called saints (1 Cor. 1:2). Every-believing sinner is, at conversion, cut off from his old standing in Adam and blessed in Christ. He is set apart from the position of guilt and condemnation in which he had been and is in Christ where there is no condemnation (Rom. 8:1). Let us be clear on this point. Read Heb. 10:10-14. It is here stated in the clearest of terms that by the one perfect sacrifice of the cross believing sinners are once and for all and for ever sanctified. To this sanctification nothing can be added nor subtracted. It is as complete at the moment of conversion as it will be in the eternal glory. By the death of Christ, who suffered without the camp that He might sanctify the people by His own blood, sin has been put away from before God and sins have been borne away from the believer (Heb. 13:12; 9:26, 28). The believer is sanctified in Christ Jesus and there can be no more question of guilt, or judgment or penalty. He has been separated from these by the cross and has acquired a perfect standing before God, accepted in the beloved (Eph. 1:6). He has been set apart for God from a world that crucified and rejects Christ. He died to its abuse and its applause, its religion and its politics. This sanctification is the common lot of every believer, of the babe in Christ and of the mature man of God.
SANCTIFICATION AFTER CONVERSION
It must be kept in mind however, that sanctification is not only positional but practical, not only of standing but of state. It is of this latter that Heb. 12:14 speaks: "Holiness, without which no man shall see the Lord." Unless there is some measure of practical sanctification, separation to God from evil, there is no salvation. Therefore Peter exhorts: "As he which has called you is holy, so be ye holy in all manner of conversation" (1 Peter 1:15). This practical holiness is in all manner of living. It is to reach spirit, soul and body (1 Thes. 5:23). It affects moral conduct (1 Cor. 6:11) even in the intimacies of domestic life (1 Thes. 4:1-7). It involves separation from filthiness of the flesh and spirit, from moral and religious evil (2 Cor. 7:1).
The word of God in the power of the Spirit is the means of producing this holiness. The Lord Jesus prayed: "Sanctify them through thy truth, thy word is truth" (Jn. 17:17). And He sanctifies His church by the word (Eph. 5:26). Psalm 119:9 teaches that it is by heeding the word that the way may be cleansed. The reading, meditating and studying of Scripture carried on daily and continually will purify the life of the believer and make him to be a vessel unto honour, sanctified, and meet for the Masters use (2 Tim. 2:21). The goal is, to be sanctified wholly (1 Thes. 5:23). Obedience to the instructions of the Lord will result in "perfecting holiness in the fear of the Lord" (2 Car. 6: 14-7:1). It is no matter of boasting and empty talking. Sanctification is separation from evil unto God, and full sanctification would be perfect separation from evil and perfect dedication to God.
It is evident then that sanctification means neither the improvement nor the extinction of the old sinful nature. The former is impossible and the latter will not take place while we are in this mortal body. Before conversion the believer had one nature wholly sinful. Since, he has two natures, one still wholly sinful, the other completely holy. In eternity he will only have the latter. Meanwhile he is to mortify his members which are upon the earth (Col. 3:5) and fortify the new nature (1 Peter 2:1-2).