Atonement is an effort to make up for our wrongdoing. Since the fall of Adam and Eve mankind had been aware of the need to make up or to be forgiven for its sins before God.
But, how could this be accomplished? Since sin deserved death, Abel sacrificed the firstling of the flock while his brother Cain offered the fruit of the soil. Only Abel's sacrifice became acceptable to the LORD thus indicating the sacrifice of blood.
In the Hebrew religious culture, the Atonement Sacrifice became the "feast" Yom Kippur. On that day the people had to fast and abstain from work.
It was the duty of the High Priest to offer this sacrifice once a year as described in Leviticus 16. The High Priest dressed in a linen robe sacrificed a bull. He entered the Holy of Holies where the ark containing the Ten Commandments was covered with the Mercy Seat, a cover with two golden angels facing each other. In the space between the wings resided God's presence. The Holy of Holies signified the separation that existed between God and man. The High Priest entered the Holy of Holies where he burned incense and with his fingers sprinkled some of the bull's blood on the Ark and Mercy Seat. This was done in Atonement for his own sins and those of his family.
Next, a goat was sacrificed whose blood was also sprinkled on the Ark and Mercy seat in Atonement for the sins of the people. This was followed by placing his hands on the head of the second goat and confessing over it all the sins of the people. It was then driven out into the desert. The High Priest than put on his normal vestments and completed the offering of the bull and goat by burning them outside the camp. These sacrifices indicated that a substitute was needed to atone for man's sins.