I heard many years ago that the entire Bible could be summed up in two short passages. The first is Genesis 22:7 when Isaac says to his father Abraham, "where is the lamb?" Indeed, where is the lamb, the sacrifice, that thing that will make us right before a Holy God? The Old Testament, at it's heart, is there to show us that we have a need and we can't meet it. The second passage is John 1:29 when John the Baptist says of Jesus, "Behold, the Lamb of God." The perfect sacrifice. God provided Abraham with a ram to offer in place of Isaac. God commanded Aaron to offer a goat to atone for Israel's sin. The atonement for sin had to be made over and over again. It was what God commanded but it was not perfect. It wasn't supposed to be perfect. In fact it was supposed to be imperfect, lacking, insufficient so that Israel would be watching for the promised Messiah who would make the perfect sacrifice once for all. The Old Testament shows our need, the New Testament shows God's perfect provision.
"Behold, the Lamb of God, who takes away the sin of the world!" John 1:29