To sanctify means to set something apart for sacred use. It becomes set aside for holy use, therefore it becomes holy itself. When you get saved, you are considered forgiven of all sin and you are sanctified before God, and then you must become what you are. Though set apart as holy for God’s use, we all still have a sin nature to contend with, and we will still need forgiveness for our failings and help resisting temptation.
Sometimes I think it would be great if when the Holy Spirit came to live within us God just fixed everything right up entirely and we became immediately perfect, but really that would take away our volition. God gave us volition on purpose, so He’s not just going to take it away. He does much better, He teaches us how to use it properly! But our problem is that we often won’t use our volition properly, and it takes a lot of work to get us to consistently let God work in and through us.
It’s like letting Jesus into your house and then bargaining with Him about which rooms over which He can be the Lord, except Jesus really owns the whole house. (Reminds me a little of 1 Corinthians 6:19-20.) “Jesus, you can have the living room and kitchen, but I’d like to keep this closet just for me. And maybe also my bedroom,” we say, when it isn’t really ours. Jesus of course doesn’t force His way in, but He does work with us on our resistance and it does take effort from our volition. We’ve got some house cleaning to do. God is fully in charge and has full ownership, yet He engages my volition, engages me, to work together with Him in sanctifying the house.