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.
In Isa. 66:17 some sanctified themselves to eat swine's flesh.
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. 15:16; 1 Cor. 6:11; 2 Thess. 2:13; 1 Peter 1:2. See also Acts 20:32; 26:18; 1 Cor. 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; 13:12 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. 10:29. 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. 7:14 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 10:36, He sanctified Himself, that is He set Himself apart on high in the glory John 17:19 to be the object of His own that they might sanctify themselves by this truth. So in 2 Cor. 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 1:16; 2 Cor. 7:1; 1 Thess. 4:3. "Follow... holiness" in Heb. 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. 5:23; Eph. 2:10; Phil. 2:13; 1 Cor. 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.
Even our food is "sanctified by the word of God and prayer" 1 Tim. 4:4, 5.
Holiness
Holiness is a nature that delights in purity and repels evil. God is ever holy. The Lord Jesus was holy Luke 1:35; Mark 1:24; 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. 3:1, but called upon to be practically holy because God is holy.
Perfectionism
There is no such thing as perfection in the flesh Rom. 7:18; 8:7. "If we say that we have no sin we deceive ourselves" 1 John 1:8.
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 1:23; 2 Peter 1:4, which cannot sin 1 John 3:9 and by walking in the Spirit we do not fulfill the lusts of the flesh, the old nature Gal. 6:16. Thus the Christian has two distinct natures. The allowance of one disallows the other.
Two natures are seen in John 3:6; Rom. 7:22, 23, 25 and many other scriptures. "That which is born of the flesh is flesh and that which is born of the Spirit is spirit."