For by a single offering he has perfected for all time those who are being sanctified.
Sanctification is the process of being made holy, of shedding sin, of becoming set apart from the world, of being made like Christ. Through sanctification we are being made perfect.
So in Hebrews 10:10 we are told that God has perfected those who are being perfected. It is as if God says, “I have done what I am doing.” Here God shows us two sides to our sanctification. When we were saved, we were covered with the righteousness of Christ. We were instantly made holy, made so that our sins no longer separate us from God.
But while this perfection dealt with the problem that separated us from God, it did not change our daily actions. Following salvation, we continue to struggle with sin. So in addition to perfecting us with the righteousness of Christ, God continues to make us holy by growing us each day to be more like Jesus.
This means our holiness does not happen overnight. Although we have been perfected, we are still being perfected and we will continue being perfected until the day God calls us home.
But Scripture offers a promise to the people of God. In Philippians 1:6 Paul 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. He who began this good work in your life will complete this good work. He will not stop halfway through the process and give up. He will finish what he has started.
This means that you, if you are his child, will be made like Christ. You will become completely holy. This will take a lifetime and it will not be complete until the day we go home to be with him, but we rest in the confidence that our daily struggle to walk by the Spirit is not in vain. God will finish what he has started.