Leviticus 9:7

And Moses said unto Aaron, Go unto the altar and offer thy sin-offering and thy burnt offering, and make an atonement for thyself and for the people, and offer the offering of the people, and make an atonement for them, as the Lord commanded. That was the nucleus of the idea of the sacrifices, that was the purpose of the offerings at all times, to cover the sins in the sight of God, that He might forgive them. And all this was accomplished for the believing Israelites in view of the perfect atonement of Jesus Christ, of which the blood atonement made by their priests was but a dim picture.