Atonement in the Old Testament
The Hebrew verb ‘to atone’ (kaphar) means ‘cover’, so the noun ‘atonement’ (kippurim pl.) is a form of ‘covering’. The most usual form of the word in the Old Testament is kipper (piel form, causative form, of kaphar) which means to ’cause to be covered’, ‘make covering for’.
Lev.5:18 And the priest shall make covering (KJV atonement) for him on account of his ignorance.
This is all ‘atonement’ means. It’s just a way of translating the Hebrew idea of ‘covering’ sins as an idiom for ‘forgiving’ sins. Although the idea of ‘cover’ can also be used in ‘covering’, appeasing anger:
Prov.16:14 A king’s wrath is a messenger of death,and a wise man will appease it.
The word “appease” in the above is the same as “covering” or “atonement” in the 69 OT verses which traditionally contain that word. Prov.16:14 shows, uncomfortably perhaps, the basic meaning of the word in human context. Though the “appeasing” of a king, and the “covering” for sin are somewhat different, it also shows that “covering” is not simply about “hiding” sins, but fixing.
Tyndale: first use of ‘at-one-ment’ in a translation of the Bible.
According to the OED, the English word ‘atonement’ was probably in use at least by 1513, but it was first used in a translation of the Bible by William Tyndale later in the 1500s, possibly while in exile in Antwerp . The word “at-one-ment” comes from a contraction of “at one”. So first Tyndale (1526), then the Bishop’s Bible under Queen Elisabeth 1st (1588), then the Authorised Version under King James (1611), employed the word ‘atonement’ 69 times through the OT where the Hebrew is ‘cover’. However since the original context of the Hebrew meaning, sin-covering, comes through in the context, whatever Tyndale’s intentions, the meaning ‘covering’ has attached to ‘at-one-ment’ because of the contexts of the OT verses in which his new word was used. Later, following Tyndale’s death, the noun ’atonement’, created the coinage of another new word ‘atone’ (first seen in 1574 according to Merriam Webster).
In the OT, the word ‘atonement’ doesn’t disrupt the text much, as the context of ‘covering’ is usually clear.
But ut is less easy in the NT since references to OT ‘atonement’ ‘covering’ ideas are often made briefly with little surrounding context to help us identify when a NT author is making allusion to OT ‘covering’ language in NT contexts.
In addition, first we have to take out of the way one famous ‘atonement’ verse, which actually isn’t connected with OT sacrifice at all…
Tyndale’s Romans 5:11 mistake (KJV)
Since Tyndale completed his NT first, the original use of ‘at-one-ment’ actually was not so incorrect. The problem came later; when all the 69 OT uses of atonement were applied to ‘covering’, the one NT use stuck out like a sore thumb. As a result it is an irony that the only time the KJV (following Tyndale) uses ‘atonement’ in the NT, the KJV does so in a verse which does not actually contain the word in Greek.
Romans 5:11 And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement. (KJV)
One could argue that it’s the 69 OT uses of ‘atonement’ in the KJV which are the mistake, and the 1 NT use which is correct, but fundamentally there is an error because Romans 5:11 has become dislocated from 5:10 in the KJV. The word in Romans 5:11 in the original Greek is “reconciliation” (katallage) which is the noun form of the verb “reconciled” (katallasso) which Paul has just used twice in Rom.5:10-11. These three words, “reconciled”, “reconciled” and “reconciliation”, are words for healing a dispute or disagreement — nothing whatsoever to do with OT sacrifice. In fact, the same verb is used in 1Co7 for “reconciliation” of a separated husband and wife. So in the original Greek, and in modern versions, Romans 5:11 has no connection to “atonement”, and no connection to OT sacrifice.
Romans 5:10 For if while we were enemies we were reconciled (verb) to God by the death of his Son, much more, now that we are reconciled (verb), shall we be saved by his life. 11 More than that, we also rejoice in God through our Lord Jesus Christ, through whom we have now received reconciliation (noun form). (ESV)
The first major version to correct the mistake was the RV. Today all modern versions (even NKJV) have now returned to what was the original (and correct) reading of the earliest English version — that of Wycliffe “by whom we have received now reconciling.” (1382).
However, just because Romans 5:11 KJV is a mistaken translation does not make the verse irrelevant to the subject of “atonement”. Although Romans 5:11 does not use the technical OT term for “atonement for sins”, what “atonement for sins” is ultimately about is still reconciliation — specifically the reconciling of man to God (note not God to man, since God is not astray). There are a long list of NT verses about reconciliation, and the OT “atonement” idea does actually fit into the bigger theme of “reconciliation” of man to God as one component, or one of the several pictures the NT uses to illustrate this idea. Other NT pictures also used are “redemption” (purchasing of a slave) and “ransom” (of a hostage). See links at the foot of this page.
Atone – God being merciful.
In most of the OT “atone” verses, where the Hebrew is “cover” (Hebrew kaphar) the Greek translation of the OT, the Septuagint, uses “proptiate” (Greek ilaskomai). This word provokes in pagan usage is often used of ‘appeasing’ an angry God, but in OT usage and NT usage that idea is not present, since the concept is that it is man’s own actions which estrange him from God. Man needs to be reconciled, not God. And the basic concept of atonement is covering for sins coming from merciful God, not an angry God being appeased or bought off.
Luke 18:13 …. God be merciful (ilaskomai) to me a sinner.
Hebrews 2:17 Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation (ilaskomai) for the sins of the people.
In this first NT example note that Jesus is prefigured by the OT priest. He is ‘making cover for sins’. This is the first of several NT allegories built on OT atonement.
Another allegory in the NT comparing the OT practice of ‘atonement’ sacrifice can be found in 1John where John takes up language familiar from OT verses like “beside the ram of the atonement (ilasmos), whereby the priest shall make atonement (ex-ilaskomai) for him.” (Numbers 5:8 LXX)
1 John 2:2 He is the propitiation (ilasmos) for our sins, and not for ours only but also for the sins of the whole world.
1 John 4:10 In this is love, not that we have loved God but that he loved us and sent his Son to be the propitiation (ilasmos) for our sins.
In this second example John is using a different allegory from Heb 2:17. This time Jesus is the sacrifice, not the high priest. But the sacrifice does not have to mechanically be exactly the same in every respect as the “ram of atonement” offered in Numbers 5, or any other OT atonement sacrifice, this is simply an illustration; John is describing Jesus’ work so his readers can begin to grasp it.
Another related Levitical image in the NT is found in the two uses of the OT term “mercy seat” (taken from Exodus 25:17-18, Leviticus 16:2, etc. LXX)
Romans 3:25 whom God put forward as a propitiation (hilasterion, mercy-seat) by his blood, to be received by faith. This was to show God’s righteousness, because in his divine forbearance he had passed over former sins.
Hebrews 9:5 Above it were the cherubim of glory overshadowing the mercy seat (hilasterion). Of these things we cannot now speak in detail.
As Hebrews says “of these things we cannot now speak in detail”. That’s a reminder that this is only a shadow and pattern of what Christ achieved. The detail can detract from the overall picture which follows
Hebrews 9:11 But when Christ appeared as a high priest of the good things that have come,  then through the greater and more perfect tent (not made with hands, that is, not of this creation) 12 he entered once for all into the holy places, not by means of the blood of goats and calves but by means of his own blood, thus securing an eternal redemption.
As Hebrews says, it’s only an foreshadow, we don’t have to go into every detail. In fact the author of Hebrews seems deliberately wary of the detail, and omits it to concentrate on the reality — Christ. There’s a warning here. For the reader it’s very easy to miss the forest (Christ) for the trees (the foreshadows in OT atonement).
Other similar topics
Reconcliliation — see Rom 5:11, 11:15, 2Co5:18-19.
Redemption — Luke 21:28 etc.
Ransom — 1Tim2:6 etc.
Only when all these different pictures and figures are put together can a balanced overall picture of Christ’s work be seen. Concentrating too much on the “atonement” illustration, from OT sacrifice, to the exclusion of the other pictures the NT uses, can lead to an overly legal view of Christ’s work.