Sanctification begins at the moment the Holy Spirit indwells you as part of the salvation experience and ends at the saint’s last breath. The process is summed up in Romans 8:29 - “For those whom He foreknew, He also predestined to become conformed to the image of His Son, so that He would be the firstborn among many brothers and sisters.” That’s it’s purpose, to take sinners and bring them up to the sinless image of Christ. Tall order to say the least. But the Spirit’s up to it. Bible.org has a good description of the process and I invite you to take a look at it. God, too, has a good, although much shorter, description of the process that goes like this: Romans 8:13 - “for if you are living in accord with the flesh, you are going to die; but if by the Spirit you are putting to death the deeds of the body, you will live.” Sanctification is the Spirit working with your conscience, your wisdom, your sense of right and wrong, to identify the sin that separates you from God - that would be all sin - and working with you to scrape it from your soul. And once this process is begun, it is carried on to completion - to the day of Christ Jesus, as Philippians 1:6 tells us: “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.”
Sanctification is the sure sign that you are saved. When 2 Peter 1:10 tells us - “Therefore, brothers, be all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall.” Peter is telling us to search within ourselves and make sure sanctification is taking place, that our sins are being relentlessly done away with, that the Word of God continues to be our guide, and is even becoming increasingly such. By doing this we affirm our salvation. Not that we can lose our salvation, but perhaps we were never saved in the first place. Remember, the parable of the sower tells us the seeds (God’s word) falls on three of the four places where it doesn’t produce lasting roots. Only on the good soil does it flourish. Make sure your salvation is growing in the good soil.