Sanctification is the growth stage for a Christian from birth to death. Our entire life as a Christian on earth = sanctification. Sanctification follows justification.
Growth requires life first then growth of that life. When any person is born again, he or she becomes a new creation, the old is behind and the new is the future.
No one can enter into sanctification until one is born again. When we become a child of God, we cross over from death to life and all of that is by the power of the Holy Spirit.
We had to have our sins removed first so that the Holy Spirit could come into our being and begin the work of sanctification which is further purification or separation from sin inside our hearts. When we are born again, we receive a new spirit which is inhabited by the Holy Spirit. He takes up permanent residence in us and begins to change us from the inside out. He does that work in our soul which is our mind, will and emotions. This work will be for a lifetime since we will never be perfect in this life, there will always be a need for improvement.
But we grow into maturity from child to adult spiritually if we walk in the leading of the Holy Spirit.
When we are born again, we receive the Gifts of the Holy Spirit, they are different for all of us, just our entire being/personality is unique. However, the objective of the Holy Spirit is the same for all Christians. We are being transformed into the image and likeness of Christ as we use those gifts. As we obey, people should see the Fruits of the Holy Spirit, that is the by-product of sanctification.
We should see change for the better not through our human effort but through surrender to the Holy Spirit. Eventually, we will die in the flesh but since the same Holy Spirit who lives in Christ lives in us, we will be raised with Christ into glory – that stage is called glorification. There are only three stages in the life of a believer – justification, sanctification and glorification in that order.
Every human being is created by God into the image and likeness of God in three parts – spirit, soul and body. Spirit is a past completed work of God when the Holy Spirit regenerated us into new life and came into our being through justification – that is a PAST work for the Christian and hopefully a future work for the non-Christian. After we are saved, the Holy Spirit begins changing our minds through teaching, understanding and obedience to the Word of God which is Christ the Head of the Body. That is the PRESENT work of God that is continuous as long as we have breath. Finally, we will be raised as the Bride of Christ into a new glorified body to live in heaven forever with Him in glory – Glorification. That is a FUTURE work of God. We will review Glorification next week.
Sanctification is extremely important to understand, it is your life as a Christian until death!
Please read this answer from Got Questions for more on Sanctification – https://www.gotquestions.org/sanctification.html