It's critical for our assurance and the Christian life to understand the relationship between justification and sanctification. Alister McGrath has a great analogy that gets at this:
The German Reformer Martin Luther taught that Christians are "righteous and sinners at one and the same time" (simul iustus et peccator). We are righteous on account of God's having clothed us with his righteousness -- but inside we are still sinners. God justifies us on the basis of his gift of righteousness, rather than on the basis of any righteousness we may possess. Although we are sinners, God is still able to renew and regenerate our fallen natures. Just as workers on a building site might cover their work with tarpaulins to protect it from the elements, so God shelters his work of renewal and regeneration within us with the external covering of his righteousness.
McGrath, "I Believe": Exploring the Apostles' Creed (IVP, 1997), 97.
Our sanctification is always a work in progress. It is messy and incomplete. Perhaps it looks more like one of those government construction projects that seems to go on forever with no end in sight - a source of irritation and disappointment more than of hope. Will the project ever be complete? Yes, it will, at the last day, when he will present us faultless before his presence will great joy. In the meantime, he has us "covered." God is protecting his own work in us (Phil 2:13) by the righteousness of Christ. He ensures that the elements will not undo his labor. He guarantees that the work can proceed uninterrupted until the wonderful day of the unveiling ceremony. No enemy can thwart his purpose. He will see to it that he is glorified in his handiwork.