It is perhaps significant that there is no obvious and complete explanation of atonement and how it functioned in the Bible. My guess is that this was because it was too obvious to the ancients to require stating. People just knew instinctively that they were in need of help and closer union with the gods/ God and that offering sacrifice was the way to make it happen. Today we look at the understanding of how sacrifice and atonement functioned in the Old Testament.
We begin by looking at a statement of how atonement did not function. In Psalm 50, we find a wry and somewhat sarcastic rejection of the idea that by offering sacrifice to God we are feeding a hungry deity (thus Psalm 50:12-13: “If I were hungry I would not tell you! Do I eat the flesh of bulls or drink the blood of goats?”) This notion of feeding the god was present in ancient paganism. In the Epic of Gilgamesh the gods were starving during the flood because no one had been offering sacrifices to them during that time, and when a sacrifice was finally offered after the flood the gods hungrily gathered around it like flies when they smelled it. But such a crude notion was never found in the Torah.
Sacrifices were gifts which a grateful and needy worshipper offered to Yahweh, according to his ability (i.e. if one was too poor to offer a lamb, a lesser offering of two pigeons would suffice; compare Leviticus 12:8), and according to his need. His need might be a simple act of gratitude, or the fulfillment of a vow (Leviticus 3:1f). Sacrifice might be offered as part of one’s cleansing, or to make restitution for an inadvertent omission; Leviticus 14:1f; 4:1f). In all cases an animal was killed and its blood poured upon the altar and a part (or the whole) of the animal was burned, ascending to God in the sacrificial smoke. Such sacrifices cleansed away sin/ ritual impurity, allowing the holy God to dwell in the midst of His people (compare the rituals for the Day of Atonement, Yom Kippur, in Leviticus 16).
As said above, the theory behind such sacrifices was never explained detail. But there are hints.
In Leviticus 1:9 a sacrifice is said to offer “a soothing aroma to Yahweh”. The phrase is also found in Genesis 8:21, where it described the sacrifice Noah offered, with the result that Yahweh promised that He would “never again curse the ground any more for man’s sake, because the imagination of man’s heart is evil from his youth”. In the words of Wenham (in his commentary on Leviticus), “Though man’s heart was unchanged in his sinfulness, God’s attitude to man altered, thanks to the burnt offering…Sacrifice is the appointed means whereby peaceful coexistence between a holy God and sinful man becomes a possibility…It propitiates God’s wrath against sin”. Thus God’s wrath against Israel in the form of a plague because of the taking of a census was averted by the offering of sacrifice (2 Samuel 24:25); and thus Job offered sacrifices for his sons to avert divine wrath for their sins and (later on) to avert the judgment due to the sins of his sinful “comforters” (Job 1:5, 42:8). In the same way, wrath came on Israel for their sins when sacrifices were not offered in the Temple (2 Chronicles 29:7-8). So (to again quote Wenham), “Peace with God is the goal of sacrifice”.
The word sometimes rendered “to make atonement” (e.g. Leviticus 1:4 NASB) is the Hebrew kaphar, sometimes rendered, “to cover”, but more accurately as “to wipe away”. It is used in 2 Samuel 21:23 where David asks the offended Gibeonites what he must do for the sin of the of Saul’s house to be wiped away (for it was because of Saul’s sin against them that Yahweh’s wrath came upon Israel). It is used in Isaiah 27:9 where God says that Israel’s sin will be wiped away if they turn from their idolatry.
The word can also mean a ransom [Hebrew kopher], something offered to turn away the full force of a penalty. Thus in Exodus 21:30 a kopher could be paid which would allow the owner of a lethally-dangerous animal to live and avoid the death penalty which he would otherwise incur. However, according to Numbers 35:31, no kopher was possible in the event of first-degree murder.
Thus the blood of sacrifice atones and wipes away sin, expiating the offense, propitiating God, bringing forgiveness. We see this in the classic passage Leviticus 17:11 which reads, “The life of the flesh is in the blood, and I have given it to you upon the altar to make atonement [Hebrew kaphar] for your souls, for it is the blood that makes atonement [Hebrew kaphar] for the soul.”
It is for this reason that the Ark’s lid was called the kapporeth, usually translated “mercy seat”. This lid was the place where atonement and the wiping away of sin took place. (The idea of it being a “seat” came from such passages as Psalm 99:1, where Yahweh is said to be “enthroned above the cherubim”—i.e. above the carved representations of the cherubim that formed part of the Ark’s lid.).
The Septuagint renders kapporeth as ἱλαστήριον/ ilasterion, the place of expiation. The Greek word ἱλαστήριον is also used in 4 Maccabees 17:22, where it describes how the blood of the Maccabean martyrs functioned as an expiation for the sins of Israel. The verb form is ἱλάσκομαι/ ilaskomai, meaning “to expiate, propitiate, conciliate, forgive”. It is used in Lamentations 3:42, where it says that God punished Jerusalem, since He was not propitiated, and in Psalm 79:9 where it refers to God expiating and forgiving Israel’s sins. In Luke 18:13 it is used in the publican’s prayer that God forgive and be propitious to him despite his sins. Rendering it simply as “having mercy” or “forgiving” (apart from propitiation) betrays an inadequate understanding. The mercy and forgiveness were only possible because God had allowed Himself to be propitiated—as was apparent from its use in 4 Maccabees 17:22 where the shed blood of the martyrs functioned as a propitiation and expiation.
In summary, the word often rendered “make atonement” indicates the expiation of sin which allows God to forgive and pass over one’s offenses. Suggesting that it refers to a free and simple forgiveness and contrasting it to notions of expiation (however handy it may be to Orthodox apologists seeking to contrast a kinder Orthodoxy with those unkind Protestant views of sacrifice as propitiation) is unsound.
We may still ask: why was sacrificial blood required? Leviticus 17:11 says that life was in the blood, leading one to surmise that the life or blood released through the death of the victim could be applied to the one making the sacrifice. Sin required the death of the sinner, whereas life liberated through sacrifice could provide for the life otherwise forfeit.
In the words of a very old writer (Leighton Pullan, in his 1907 work The Atonement), “The sin of the offerer was not regarded as transferred to the animal…God accepts the life of the animal instead of the life of the offerer, the life of the animal being given to God through the blood in which the life resides. God graciously accepts the sinner in one way instead of another. In place of his actual obedience, and in spite of his sin, God accepts him in his offering which expresses his intention of obedience”. In other words, in the Old Testament the blood and life of the sacrificial animal was accepted as a payment for the debt of sin. With the offering of sacrifice, the sinful worshipper was forgiven, his sin expiated, and God propitiated. Peace could then reign between God and His people.
But one might ask another question: How could the blood of bulls and goats and the life liberated by their death suffice to eliminate sin? Does not sin which taints and contaminates the human heart require more than mere animal life for its healing? The Christian answer is: “It is impossible for the blood of bulls and goats to take away sins” (Hebrews 10:4). These might suffice for a merely ritual outer cleansing of the flesh, but to truly wipe away sin from the human heart and cleanse the worshipper, something more is required. The sacrifices of the Old Testament were expressions of the human heart’s need and desire for cleansing, but they could not themselves cleanse the heart. The most they could do was act as prophecies and promises of the desired remedy. That final remedy would wait until the Son of God hung on a cross on Golgotha.