The altar here is the golden altar of incense, which was closest to God's presence. It was located within the Holy Place, and just outside the Holy of Holies. God commanded the high priest to cover the altar's horns once a year with the blood of a special offering to purge the uncleanness. But this sin offering also is not the typical one we saw in Leviticus 4. This offering has a name that sets it apart. It is called, the sin offering of atonements or for atonements. It is also referred to in Exodus 29 and Numbers 29. Now, the Hebrew word for atonement in that name is plural because multiple objects were cleansed on this Day. Similarly, in the three places where the phrase, The Day of Atonement, is used, the word for atonement is also plural. So, it would be technically correct to call this holy day the Day of Atonements (Leviticus 23:27-28; 25:9).
So, the horns of the altar had to be purged by blood once a year using the sin offering of atonements. But not just any blood would suffice because of how God views blood and sin. Please turn to Deuteronomy 12:
Notice that even when an animal is dead, God still considers the blood to be the life of the animal. In this verse, the meat is lifeless, and yet even then, the blood still represents the life. Blood is a symbol of the life, even after it is no longer within a living animal's body.
Now, let's carry this through. In a typical sin offering, the transgressor symbolically transfers his sin by laying his hand on an innocent substitute. The animal's blood, then, which represents life, becomes a representation of the sin being atoned for. In type, the blood becomes a record of the sin.
This ceremonial aspect of blood can be seen by comparing the instructions for the sin offering with other blood sacrifices. We won't turn to it, but in Leviticus 6:27, it says that if any of the blood of the sin offering was sprinkled or splattered on the priests' garments, they had to be washed. However, this was not necessary if the blood of either burnt offerings or peace offerings got on the priests' clothes, because those sacrifices did not involve sin. The blood from those sacrifices was not considered to be defiled. But in a sin offering, the life and the sin of the guilty party were transferred to the substitute, and since the blood is the life, the blood of the substitute was symbolically defiled.
Now, remember that the blood of the typical sin offering was placed on the horns of the altar. Because of what the blood represented, it is as though the horns of the golden altar became a repository for the sins of the priests and the congregation (see Leviticus 4:7, 18). The prophet Jeremiah describes all the accumulated blood and sin:
This describes just how sinful Judah was, and God says it is like the sins are engraved on the horns of the altars. With each sin offering, another record of guilt was added to the horns. The word engraved indicates a great deal of repetition. That's a lot of sin offerings. God's prescription was an annual cleansing of the golden altar, and specifically the horns, where the defiled blood was put. But the high priest had to use blood from an animal that did not have sins transferred to it. A typical sin offering would just add more sin to the altar. Instead, undefiled blood was needed. Once a year, then, the special sin offering of atonements was made to cleanse the horns of the golden altar.
