And at the end of eight days, when he was circumcised, he was called Jesus, the name given by the angel before he was conceived in the womb.
The doctrine of the imputed righteousness of Christ onto the believer is a crucial but, unfortunately, a little known doctrine to most modern American evangelical Christians. It is one of the major doctrines that separates the Roman Catholic Church from historical Protestantism and Evangelicals. Roman teaches that Christians receive a "infused" righteousness from God. Protestants teach that Christians receive an "imputed" righteousness from God. What is the difference?
Infused righteousness is a type of synergism. "Synergism" is the working two or more parties together. Rome teaches that God does impart grace to the believer, but the believer must work with God to maintain that grace. To it's credit, Rome does not teach the crass type of "works righteousness" that most modern Evangelicals credit to it, but that is what makes it's teachings more dangerous to those who adhere to it.
Imputed righteousness is a type of monergism. "Monergism" is one working alone. Jesus Christ did all of the work that was needed for our salvation. Not only did He die on the cross to pay for ours sins and appease the wrath of God for all who would put their trust in Him, but he also lived a life in which he fulfilled all of the righteous demands of the Law. As a Jew, He obeyed every bit of the "Ten Commandments" and also of the other laws that God gave to Israel on mount Sinai. He was circumcised. He observed all of the festivals. He never even had one sinful thought.
Did you know that we have to be perfect in order to get to heaven? "How can that be?" you might ask.
This can only be made possible when we are clothe in the righteousness of another. Martin Luther called this an "alien righteousness."
Some have explained to doctrine justification with the phrase "Just as if I never had sinned". This is true about that doctrine, but it is only half of it. The Law demands obedience. When a person puts their trust in the complete works (His passive and His active obedience to God the Father), not only are their sins payed for, but also they are clothe with the righteousness of Christ. Christians are only accepted by God because of what Christ has already done for them.
The question might be asked, "How is all of this practical to my every day Christian walk?" The Gospel is not something that needs to be told to unbelievers. Christians need the Gospel preached to them, and too themselves often. Just remember when we do sin (and we will [c.f. 1 John 1:10]) God sees us as believers clothe in the righteousness of His Son. Oh, and even as Christians, our "good works" are tainted by sin. The puritan writer John Bunyan ("Pilgrim's Progress) once quote as saying that there were enough sins in one of his prayers to condemn everyone who has ever lived to hell. Again, the only reason God accepts believers is because He sees us clothe in the righteousness of His Son, the Lord Jesus Christ, our righteousness.