The Hebrew and Greek words for sanctify, holy and saint mean 'to set apart.' (Gadesh, gadosh, godesh, hagiago, hagios.)
In the Old Testament, days, mountain, tabernacle, vessels, temple, house, fields, priests, Levites, the people and their firstborn were sanctified, yet this had nothing to do with essential holiness. Without any inward change they were simply set apart for God.
Besides this the priests, Levites and people were often called upon to sanctify themselves preparatorily. They were to be separated to God from everything unsuited to Him.
All Real Christians Are Sanctified Once for All
In Christianity all true believers are saints (hagios)—set apart ones—have been once for all sanctified by the Holy Spirit Rom. 15:1616That I should be the minister of Jesus Christ to the Gentiles, ministering the gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Ghost. (Romans 15:16); 1 Cor. 6:1111And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God. (1 Corinthians 6:11); 2 Thess. 2:1313But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth: (2 Thessalonians 2:13); 1 Peter 1:22Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you, and peace, be multiplied. (1 Peter 1:2). See also Acts 20:32; 26:1832And now, brethren, I commend you to God, and to the word of his grace, which is able to build you up, and to give you an inheritance among all them which are sanctified. (Acts 20:32)
18To open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me. (Acts 26:18); 1 Cor. 1:22Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours: (1 Corinthians 1:2). Like stones quarried out of the pit they have been set apart by Another for the temple of God. This work has never to be repeated.
Positional or Provisional Sanctification
In Heb. 2:11; 10:10, 14, 29; 13:1211For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren, (Hebrews 2:11)
10By the which will we are sanctified through the offering of the body of Jesus Christ once for all. (Hebrews 10:10)
14For by one offering he hath perfected for ever them that are sanctified. (Hebrews 10:14)
29Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? (Hebrews 10:29)
12Wherefore Jesus also, that he might sanctify the people with his own blood, suffered without the gate. (Hebrews 13:12) we see the perfection of the work of Christ for all who are set apart provisionally in the new Christian position, yet, if lacking heart-faith, might give it all up as does the apostate. "Hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing and hath done despite unto the Spirit of grace" Heb. 10:2929Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? (Hebrews 10:29). This unusual aspect of sanctification in Hebrews has been seldom noticed although often found in Mr. Darby's writings.
Positional, provisional and relative sanctification is also found in 1 Cor. 7:1414For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy. (1 Corinthians 7:14) where we see the unbelieving husband or wife sanctified in the believing partner, and even includes the children of the household.
Practical Sanctification
After the Lord Jesus was sanctified by the Father John 10:3636Say ye of him, whom the Father hath sanctified, and sent into the world, Thou blasphemest; because I said, I am the Son of God? (John 10:36), He sanctified Himself, that is He set Himself apart on high in the glory John 17:1919And for their sakes I sanctify myself, that they also might be sanctified through the truth. (John 17:19) to be the object of His own that they might sanctify themselves by this truth. So in 2 Cor. 3:1818But we all, with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory, even as by the Spirit of the Lord. (2 Corinthians 3:18) "beholding the glory of the Lord" causes moral transformation by the Holy Spirit.
Gaze thou on Him, think upon Him,
Dwell upon His wondrous grace;
When the heart is filled with Jesus
Thoughts of self will find no place.
Practical sanctification is not the work of a moment but should continually engage us. So-called "second blessing" or "the baptism" is foreign to this truth, regardless of the pretentious and deceptions of many.
Since believers are set apart eternally they are exhorted to set themselves apart unto the Lord from everything unsuited to Him 1 Peter 1:1616Because it is written, Be ye holy; for I am holy. (1 Peter 1:16); 2 Cor. 7:11Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God. (2 Corinthians 7:1); 1 Thess. 4:33For this is the will of God, even your sanctification, that ye should abstain from fornication: (1 Thessalonians 4:3). "Follow... holiness" in Heb. 2:1414Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil; (Hebrews 2:14) is the character of every real Christian—separation from evil in following the Lord. As to what extent this is done, a true sense of our failures would prevent our boasting. Even this is the work of God 1 Thess. 5:2323And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ. (1 Thessalonians 5:23); Eph. 2:1010For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them. (Ephesians 2:10); Phil. 2:1313For it is God which worketh in you both to will and to do of his good pleasure. (Philippians 2:13); 1 Cor. 1:30, 3130But of him are ye in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption: 31That, according as it is written, He that glorieth, let him glory in the Lord. (1 Corinthians 1:30‑31). The evidences are seen in what others behold as to our homes, our dress, our pursuits and our associations. Only real going on in heart enjoyment, communion and subjection to the Lord will demonstrate our practical separation from the spirit of the world.
Holiness
Holiness is a nature that delights in purity and repels evil. God is ever holy. The Lord Jesus was holy Luke 1:3535And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God. (Luke 1:35); Mark 1:2424Saying, Let us alone; what have we to do with thee, thou Jesus of Nazareth? art thou come to destroy us? I know thee who thou art, the Holy One of God. (Mark 1:24); Acts 3:1414But ye denied the Holy One and the Just, and desired a murderer to be granted unto you; (Acts 3:14). "Partakers of His holiness" (hagiotees is found here only and is the quality itself). Christians are called holy because set apart for God absolutely Heb. 3:11Wherefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our profession, Christ Jesus; (Hebrews 3:1), but called upon to be practically holy because God is holy.
Perfectionism
John Wesley derived his system of doctrine from Bishop Taylor, Law and Thomas a-Kempis (catholic) and then tried to make scripture square with it. These mystics piously endeavored to love God as though He was requiring it instead of producing it by the full knowledge of His own love and grace in the gift of Christ. They failed to see Christ's unfallen holy human nature. They specialized in human attainment unto sinless perfection so they could be satisfied with themselves to the point of glorying.
They failed to distinguish between sins (fruit) and sin (the root). Eternal relationship is confounded with our varying states of soul.
They misuse scripture as to the word ‘perfect’ where it has nothing to do with the root of sin in our old nature. 'Perfect' sometimes means 'mature' and in Hebrews generally refers to resurrection glory.
Improving Adam is not Christianity. Christ cannot be improved. "In Him dwelleth all the fullness of the Godhead bodily, and ye are complete in Him." "Accepted in the Beloved" "Christ is everything" "Altogether lovely." Perfection is found in Him alone now in the glory. He is "our life" our glory, hope and goal.
Two Natures
Young Christians are cast down at the discovery of failure upon failure within themselves.
New birth is not the changing of the fallen nature nor the eradication of it.
We have a distinct new nature 1 Peter 1:2323Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever. (1 Peter 1:23); 2 Peter 1:44Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust. (2 Peter 1:4), which cannot sin 1 John 3:99Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. (1 John 3:9) and by walking in the Spirit we do not fulfill the lusts of the flesh, the old nature Gal. 6:1616And as many as walk according to this rule, peace be on them, and mercy, and upon the Israel of God. (Galatians 6:16). Thus the Christian has two distinct natures. The allowance of one disallows the other.