Sanctification is the ongoing process by which the Spirit of God conforms or molds a believer into the image of Jesus Christ. Where Justification is the one time act whereby a person is made right before God, sanctification continues from the moment of regeneration and justification until death. This process involves both the individual who has the Spirit within them and the Spirit Himself. The Spirit works within the individual to will and to do of His good pleasure, conforming us to the Son’s image day by day. The individual strives, by the power of the Spirit within them, to resist sin and to live in a manner worthy of the calling to which we have been called.
First Corinthians 6:11 says, “And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.” This is a continued thought from the preceding verses in which Paul labels what men used to look like prior to the life-changing act of regeneration. The main phrase here is, “and such were some of you.” Through the process of sanctification, the Spirit leads us into all holiness and Godly living. We are not who we used to be, nor do we continually desire the things that we used to desire. Second Corinthians 5:17 says, “Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.” Once we are justified by the perfect sacrifice of Jesus Christ, we are the new creation that Paul writes about. Our old selves have passed away and we are made new.
The struggle that we have as believers is that we are not yet glorified. Because sanctification is an ongoing process, we continue to struggle day by day as our flesh battles against the Spirit within us. This side of Heaven, we will not be perfect nor will we completely defeat sin in our own lives. However, because it is the Spirit of the Living God that lives within us, day by day, we will be pressed toward the goal of perfection, being sanctified by that very same Spirit. Paul writes in his letter to the church at Philippi, “And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ.” Paul was certain that the very change wrought in our hearts by the Holy Spirit would be brought to completion. This insinuates an ongoing process that leads to completion. It does not mean that we have been made perfect, nor will we be in our current state. However, what God began in us, He will carry it on to completion.
We also see in Paul’s writings that sanctification is not just an option tacked on to the process of salvation. It is, in fact, what God has foreordained for every believer and it will take place in every believer. Paul says in his book to the church at Rome, “for those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers.” Paul proves that sanctification is not just an accident, but it is the very will of the Father to conform His children to the image of His Son.
 Phil 1:6 (English Standard Version)
 Rom 8:29