We are already perfect, yet we are being made holy. It takes time for a prince to stop acting like the frog it used to be. But I will not drive them out in a single year, because the land would become desolate and the wild animals too numerous for you. Little by little I will drive them out before you, until you have increased enough to take possession of the land. Little by little God will drive out the enemy who has kept us tangled in sin.
Sanctification comes a step at a time.
After receiving Jesus as our Lord and Savior, we still need to work with the Holy Spirit to overcome areas of sin. Sanctification is a lifelong process which is completed when we die and go to be with Jesus. We continue to become more and more like Jesus as we cooperate with the work of the Holy Spirit in our lives. The analogy of a seed works well here. When Jesus died on the cross, His righteousness became available to us.