“And 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, KJV)

To sanctify means to set apart or to make holy.
A believer is instantly sanctified in the moment that salvation occurs. First Corinthians 6:11 tells us that the believing sinner is washed, sanctified, and justified. There is no such thing as a believer who is justified and washed in the blood who is not legitimately sanctified in the eyes of God.
This is instant sanctification.
To reinforce the thought, it is provided at redemption. After this, the believer stands in need of being progressively sanctified. This pertains to the believer’s spiritual growth. The end result is that the believer is conformed to the nature and character of Christ.
The term sanctification is often applied to either sense of the word—instant and progressive—and has caused confusion and much debate over the years.

As stated earlier, instant sanctification occurs at salvation.

One might view it as a legal position in Christ, given on the basis of faith in Jesus as Savior. By virtue of the sacrifice of Christ, the new believer is set apart from sin and to God. Instant sanctification is necessary in order for God to establish a relationship with the new believer. If this were not so, the Holy Spirit could never make His home in the heart of the new Christian. So positionally, instant sanctification provides God with the means of obtaining and maintaining a personal relationship with the believer. The believer has been called out of the world and its sinful system and dedicated to a God who thinks and operates far differently than those that are active in the world.

It is God’s intention to transform the new believer into His own image. But that progression would be impossible without the status of sanctification first being applied to the heart and life of the believer. Again, this position is made available by the individual simply placing his faith in Christ and His finished work.

For every person that accepts Christ as his Savior, there is the responsibility to be conformed to the image of Christ. There is no event that can provide us with this transformation.
Being conformed to the image of Christ demands the consistent work of the Holy Spirit. He is the sole sanctifying agent in the heart and life of the believer.
The simple truth is everyone who has been saved is the recipient of the Holy Spirit. He is entirely able to conform the believer into the image of Christ from the moment of salvation forward. We must believe that salvation provides the believer with the sanctifying help of the Holy Spirit. If a believer will trust and depend upon what Christ has accomplished for us on the Cross, the Holy Spirit will continuously glorify Christ by conforming that believer into the image of Christ.

It is faith in Christ and Him crucified that gives the Holy Spirit the legal right to continually work in us and change us nearer to the character and nature of our heavenly Father. Instant sanctification occurs at salvation and effects our position and standing with God. Progressive sanctification is an ongoing necessity of the Christian experience that starts the moment the believing sinner comes to Christ.
It will not be completed until the death of that believer or until the return of Christ.
Progressive sanctification is not accomplished by disciplines, laws, or religious routines. While all these have their place in the Christian experience, none cause the Holy Spirit to labor on our behalf.

Faith in Christ and what He has done for us at Calvary is the key to experiencing sanctification, both instantaneous and progressive.