Perhaps no day on the Jewish calendar was more important that the Day of Atonement. It was a day which the book of Leviticus centers around. And yet, it was not sufficient to fully take care of mankind’s sin problem. That’s why Christ came, to be our sacrifice. The Lamb of God came to take away the sin of the world.
Sin has created a separation between God and man, and no work of mankind can every bridge that gap. But God has made a way, through Christ, for us to be reconciled to him, for us to recieve atonement and be made “at one” with him.