Justification is the act of God by which he declares us to be just or righteous or perfect because by faith alone we have been united to Jesus Christ, who is perfect, who is just, who is righteous. So, justification is a legal standing before God, owing to a spiritual union with Jesus, which is owing to faith alone. You don’t work yourself into or perform your way into this standing with God. He declares you to be perfect because of your union with Christ, and that happens by faith alone.
Sanctification is the act of God by which he, through his Spirit and his Word, is conforming you little by little—or in big steps—into the image of his Son. So we are really becoming in our behavior righteous, really overcoming imperfections in our sanctification.
Now here’s the key question: How do these two relate to each other? The key verse is Hebrews 10:14: “By a single offering, [Christ] has perfected for all time those who are being sanctified.” Think of what that says. Who has been perfected for all time? Has been. It’s done. Has been perfected for all time. Those who are being perfected. Being sanctified. Being made holy. He has made you perfectly holy. Who? The ones who are becoming holy. Which means that the evidence that you stand holy or perfect or just before God is that you are by faith becoming holy. Sounds kind of paradoxical, I know. But it’s the key to the Christian life.
Another way to say it is like this: The power by which you daily strive to overcome the imperfections in your life is the confidence that you’re already perfect. If you get these switched around, if you think, “Okay, God demands perfection; I’ve got to become in my behavior perfect, and then God will look at me and say, ‘He’s doing pretty good; we’ll let him be perfect or count him to be perfect.’” It’s just the opposite. Because of Christ, we believe in him and what he did on the cross and his perfect life. We believe in him, and by that faith, God unites us to Christ. His perfection is counted as ours. And the evidence that we stand perfected in Christ is that we hate our sin, and we daily, by faith in his promises, strive to overcome the imperfections that exist.
So my exhortation would simply be, please don’t get these backward. The whole world gets it all backward. Other religions get it all backward, where our works and our efforts to overcome imperfections might make us pleasing to God. You never can get there that way. God reckons us as acceptable, makes us his children, counts us as righteous; and because of that righteousness we then spend a lifetime becoming what we already are.