God made him who had no sin to be sin for us, so that in him we might become the righteousness of God.
(2. Corinthians 5:21)
One of the big difference between the Old and the New covenant is how righteousness is obtained.
Under the old covenant it was something you worked for by living right and offering sacrifices according to the law. The problem was that nobody could was able to do this as perfectly as a perfect God required.