I've taken the title "Sanctification of the Spirit" from Paul's words in 2Thes 2:13. "…God hath from the beginning chosen you to (unto) salvation through Sanctification of The Spirit …"

For many the Bible doctrine of sanctification is thought to be a negative matter; "Don't do this" and "Don't do that." Paul here speaks against this form of false sanctification as fostered by the "doctrines of men," while not being of God.

21 Do not handle [this], Do not taste [that], Do not even touch [them], 22 Referring to things all of which perish with being used. To do this is to follow human (manmade) precepts and doctrines. Colossians 2:21-22 (AMP)

No such human effort or law-keeping will make us spiritual, holy or sanctified. True Biblical sanctification is not a negative matter at all. Rather it is the positive truth that God wants us for Himself, to enjoy a living union with Him by "the Spirit of life in Christ" indwelling our spirit so as to flow out through us as He wills.

Biblical Sanctification Is A Twofold Truth:

Biblical sanctification means "to be set-apart as being holy." The Holy Spirit of God is the agent of the believer's sanctification, which has two aspects.

1) There is Sanctification Relating to Our Eternal Standing "In Christ," before God, as being set-apart once for all time, as a child of God the Father.

3 May blessing (praise, laudation, and eulogy) be to the God and Father of our Lord Jesus Christ Who has blessed us in Christ with every spiritual (given by the Holy Spirit) blessing in the heavenly realm! 4 Even as [in His love] He chose us [actually picked us out for Himself as His own] In Christ before the foundation of the world, (so) that we should be holy (consecrated and set apart for Him) and blameless in His sight, even above reproach, before Him in (His) love.
Ephesians 1:3-4 (AMP)

2) Then there is Sanctification Relating to Our Present Practical Varying States Of Being in our daily living in these bodies of human flesh.

Let's now consider both of these aspects of sanctification in more detail.

1. Sanctification Is Our Standing Before God "in Christ"

In this first sense of sanctification every true believer in Christ has already been sanctified, set-apart, cut out of the flock of humanity and consecrated to God, by the operation of the Holy Spirit. This is our unshakable standing of being set-apart, being "in Christ," before God. Thus Paul wrote:

"…God hath from the beginning chosen you to (unto) salvation through (or by) Sanctification of The Spirit…" (2 Thes. 2:13).

Now look at this same verse from the Amplified Bible to get a more expansive and better understanding of the Spirit's role in our being sanctified or set-apart.

13 ... God chose you from the beginning as His firstfruits (first converts) for salvation Through The Sanctifying Work Of The [Holy] Spirit .... 2 Thes 2:13 (AMP)

Then also look at this verse that says essentially the same thing.

2 Who were chosen and foreknown by God the Father and Consecrated (Sanctified, made holy) By The Spirit to be obedient to Jesus Christ... 1 Peter 1:2a (AMP)

This first sanctification has nothing to do with our conduct. It is God who has sanctified us. Sanctification always begins with Him. Thus Paul could write to even the careless fleshly Corinthian believers, saying:

"Ye ARE (already) sanctified" (1 Cor. 6:11; cf. Acts 20:32; 26:18), i.e., "God has (already) set you apart for Himself."

This first phase of sanctification is based solely upon the redemptive work of Jesus of Nazareth at the cross in our behalf. Thus this is an established unchangeable fact from the moment we first trust Christ as Savior, by believing in His death, burial, and resurrection according to Paul's gospel, as written in 1Cor 15:1-4).

"We ARE sanctified through the offering of the body of Jesus Christ once for all." Heb. 10:10

2. Sanctification Related to Our Present State Of Being

God would also have us appreciate the first fact of Sanctification as being our present standing before God... and then conduct ourselves accordingly, consecrating ourselves to trust Christ in us ever more completely as He leads us in our practical sanctification. This Sanctification is the practical and progressive Sanctification as seen in the daily walk believer.

"For this is the will of God, even (that is) your sanctification" (I Thes. 4:3).

Hence Paul's made these statements.

"The very God of peace sanctify you wholly." Then also note that Paul exhorted Timothy to be "a vessel unto honor, sanctified, and meet [fit] for the Master's use" (2 Tim. 2:21 KJV).

How can believers be more wholly or completely sanctified to God in their practical experience?

1. First, sanctification comes by hearing, reading, studying and meditating upon His written "Word of truth." Our Lord prayed:

"Sanctify them through Thy truth: Thy Word is truth" (John 17:17 KJV).

Paul declares that 25 ...Christ "gave Himself up..." at the cross so 26That he (Christ) might sanctify and cleanse it (the Church) with the washing of water by the word, Eph 5:25b-26 (KJV). God's "Word" is sanctifying water as well as food for us. The Bible tell us His "Word" is even above His Holy name (Psalm 138:1-2).

2. Then also we are to be sanctified by the Word of Christ alive within us, which is by "the Spirit of life in Christ Jesus" dwelling within us. In the course of our daily walk it is first the normal innermost speaking of "the Spirit Christ" that works to guide and regulate us; sometimes prompting or restraining us, while empowering us ... from within.

2 For the law (spontaneous regulating operation) of the Spirit of life [which is] in Christ Jesus [the law of our new being]... has freed me from the law (operation) of sin and of death (in our flesh. Romans 8:2 (AMP)

13 [Not in your own strength] for it is God Who is all the while effectually at Work In You [energizing and creating In You the power and desire], both to will and to work for His good pleasure and satisfaction and delight.
Philippians 2:13 (AMP)

Thus we see that our sanctification is by God's work by Christ cross-work for us and then also by Christ's activity within us.

Sanctification is part of the inheritance we have "in Christ."

11 In Him (Christ) we also were made [God's] heritage (portion) and we obtained an inheritance; for we had been foreordained (chosen and appointed beforehand) in accordance with His purpose, Who works out everything in agreement with the counsel and design of His [own] will, Ephesians 1:11 (AMP)