This excerpt is from R.C. Sproul’s The Truth of the Cross. Original Source: http://www.ligonier.org/blog/jesus-died-he-lived/

We must see that the righteousness of Christ that is transferred to us is the righteousness He achieved by living under the Law for thirty-three years without once sinning. Jesus had to live a life of obedience before His death could mean anything. He had to acquire, if you will, merit at the bar of justice. Without His life of sinless obedience, Jesus’ atonement would have had no value at all. We need to see the crucial significance of this truth; we need to see that not only did Jesus die for us, He lived for us.

Roman Catholics call this concept a legal fiction, and they recoil from it because they believe it casts a shadow on the integrity of God by positing that God declares to be just people who are not just. In response, the Reformers conceded that this concept would be a legal fiction if imputation were fictional. In that case, the Protestant view of justification would be a lie. But the point of the Gospel is that “imputation is real—God really laid our sins on Christ and really transferred the righteousness of Christ to us. We really possess the righteousness of Jesus Christ by imputation. He is our Savior, not merely because He died, but because He lived a sinless life before He died, as only the Son of God could do.

Theologians like to employ Latin phrases, and one of my favorites is one that Martin Luther used to capture this concept. The essence of our salvation is found in this phrase: Simul Justus et pecator. The word simul is the word from which we get the English word simultaneous; it means simply “at the same time.” Justus is the word for “just.” We all know what et means; we hear it in the famous words of Julius Caesar in the Shakespeare tragedy: “Et tu, Brute?” (“You, too, Brutus?”) Etmeans “also” or “and.” From the word pecator we get such English words aspeccadillo (“a little sin”) and impeccable (“without sin”); it is simply the Latin word for “sinner.” So Luther’s phrase, Simul Justus et pecator, means “At the same time just and sinner.”

This is the glory of the Protestant doctrine of justification. The person who is in Christ is at the very same instant a sinner and just. If I could be justified only by actually becoming just and having no sin in me, I would never see the kingdom of God. The point of the gospel is that the minute a person embraces Jesus Christ, all that Christ has done is applied to that person. All that He is becomes ours, including His righteousness. Luther was saying that at the very instant I believe, I am just by virtue of the imputation of Christ’s righteousness. It’s Christ’s righteousness that makes me just. His death has taken care of my punishment and His life has taken care of my reward. So my justice is completely tied up in Christ.

In Protestantism, we speak of this as the doctrine of justification by faith alone, for according to the New Testament, the only means by which the righteousness and the merit of Christ can come into our accounts and be applied to us is by faith. We can’t earn it. We can’t deserve it. We can’t merit it. We can only trust in it and cling to it.