The Purification for Sin Offering (Exodus 29:10-13).
“And you shall bring the ox bull before the Tent of Meeting, and Aaron and his sons shall lay their hands on the head of the ox bull, and you shall kill the ox bull before Yahweh at the door of the Tent of Meeting. And you shall take of the blood of the ox bull, and put it on the horns of the altar with your finger, and you shall pour out all the blood at the base of the altar. And you shall take all the fat that covers the inwards, and the caul on the liver, and the two kidneys, and the fat that is on them, and burn them on the altar. But the flesh of the ox bull, and its skin, and its dung, you shall burn with fire outside the camp. It is a sin offering.”
Because it is a sin offering offered for the priest it must all be consumed in one way or another. No part of it may be eaten. It is to be brought to the altar before the Tent of Meeting and Aaron and his sons will then identify themselves with the ox bull by laying their hands on it. And then the ox bull is killed as representing them. It dies that they may live. Whether they see it as signifying their sins passing from them to the ox bull, or whether they see the laying on of hands as a sign of identification, would probably depend on the offerer. The latter is certainly what the laying on of hands would signify in the future. But whatever the detailed significance the ox bull was dying for their sins. And they knew it.
And now due process must follow. The blood is first put on the horns of the altar which point heavenward. The horns were clearly seen as the most sacred part of the altar, possibly because they were nearest to heaven, or possibly because they pointed upwards, or possibly because they indicated the strong point of the altar. So the fact of the shedding of the blood is to be drawn to His attention or to be carried up to God. Then the remainder of the blood is poured out at the base of the altar. The life given in death belongs to God. All life is His, especially life given in death. So all must be His.
Then the fat and the delicate parts are burned on the altar as an offering to God. These were seen as the best parts of the sacrifice and therefore a due gift for God. They represented the thankfulness of the sinner as he was relieved of his sins. The reference to liver and kidneys is because in other nations they would often be kept aside for divination. It was not to be so with Israel. They were to be given to God.
And finally the remaining carcass, with its dung/offal (which was not seen as fit for God), is burnt outside the camp. Because the sole purpose of the ox bull is to bear their sin, its flesh, that part which is man's, is not fit for an offering, and it cannot be eaten. It must be destroyed utterly, for that is the wages of sin. And it must be burned outside the camp lest it defile the camp. Had it been seen as ‘holy' it could have been burned on the altar. No greater sense of the horror of sin and the destruction it brings is possible. But the offerer rejoices in that by the good favour of God his sin is now dealt with. He is forgiven. But in order for the whole to be effective the heart must be in it. It is only effective when the worshipper is sincere (Isaiah 1:10-17; 1 Samuel 15:22; Micah 6:7-8).