Atonement is from atone, from an early English phrase at one, in accord, (archaic at oon), short for to set at one, to reconcile, to bring together. Atonement brings about reconciliation and a restoration of unity between two formerly opposing parties where there has been a profound breach or rupture in a relationship. See Enmity.
In Christian and Hebrew doctrine atonement is the satisfaction or payment of a penalty. It can be reparation or making amends. The sacrifice of atonement is an expiation of sin: to expiate is to make atonement, to make amends, to appease the one (or the many) injured or offended, by the act of propitiation. The offense of sin and the cause of the hostility of division between God and man is removed "as far as the east is from the west" (Psalm 103:12). The sinner is wholly cleansed by the offering of the blood of the Lamb of God, and the sin is washed away, entirely removed (Leviticus 4; Numbers 19; Isaiah 1:18; John 1:29; Acts 22:16; Romans 11:27; 1 Corinthians 6:11; Ephesians 5:26; Titus 3:5; Hebrews 9:26; 1 Peter 3:21; 1 John 3:5; Revelation 1:5 and 7:14).
In the Old Testament atonement is a cover or covering for sin (Hebrew kippur; French coiffure; English coffer, cover). The sinner cannot remove the sin blot themselves, it can only be covered or atoned for. Yom Kippur, or the Day of Atonement, is the holiest day of the year in Judaism. The King James bible translates Strong's H3722 kaphar in the following manner: atonement (71x), purge (7x), reconciliation (4x), reconcile (3x), forgive (3x), purge away (2x), pacify (2x), atonement...made (2x), merciful (2x), cleansed (1x), disannulled (1x), appease (1x), put off (1x), pardon (1x), pitch (1x). Strong's H3725 kippur is translated atonement 8 times. Yom Kippur, or the Day of Atonement, is the holiest day of the year in Judaism.
The word does not appear in the New Testament because the Bible teaches that Christ washed away the sin completely. This is clearly expressed in Martin Luther's commentaries on Saint Paul's Letters to the Romans and the Galatians, and in John Calvin's Institutes of the Christian Religion. See also Calvinism. The sin is washed white as snow, blotted out, not remembered any more, taken away, washed away, passed away, entirely cleansed, purified, purged, gone forever, in Christ Jesus through the atonement in his blood. The forgiven Christian is in him, and in him is no sin. Christ died once for all (Hebrews 6:10, 1 Peter 3:18) to atone for sin. To reject this axiom is to leave a myriad of sins, both personally, and collectively of a society, unaccounted for and exposed. (Hebrews 6:4-8; 10:28-31; 2 Peter 1:9.) Roman Catholic doctrine teaches a sinner must undergo a final purging of their sin by his or her self in Purgatory, without the blood of Jesus, because the blood of Christ is insufficient to purge all sin.
Compare the following verses (KJV, boldface emphasis added):
—"As far as the east is from the west, so far hath he removed our transgressions from us."
—"Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be white as snow; though they be red like crimson, they shall be as wool."
—"And an highway shall be there, and a way, and it shall be called The way of holiness; the unclean shall not pass over it;".
—"I, even I, am he that blotteth out thy transgressions for mine own sake, and will not remember thy sins."
—"Behold the Lamb of God, which taketh away the sin of the world."
—"And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord."
—"But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life."
—"For this is my covenant unto them, when I shall take away their sins."
1 Corinthians 6:11
—"And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God."
2 Corinthians 5:17
—"Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new."
—"According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:".
Ephesians 5:25; 5:26; 5:27
—"Christ also loved the church, and gave himself for it; That he might sanctify and cleanse it with the washing of water by the word, That he might present it to himself a glorious church, not having spot, or wrinkle, or any such thing; but that it should be holy and without blemish."
1 Timothy 1:5
—"Now the end of the commandment is charity out of a pure heart, and of a good conscience, and of faith unfeigned:".
—[Jesus Christ] "Who gave himself for us, that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works."
—"Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;".
—"but now once in the end of the world hath he appeared to put away sin by the sacrifice of himself."
—"How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?"
2 Peter 1:9
—"But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins."
1 John 1:7
—"and the blood of Jesus Christ his Son cleanseth us from all sin."
1 John 1:9
—"If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."
1 John 3:5
—"And ye know that he was manifested to take away our sins; and in him is no sin".
1 John 4:13
—"Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit."
1 John 5:20
—"and we are in him that is true, even in his Son Jesus Christ." 
—"Unto him that loved us, and washed us from our sins in his own blood."
—"These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb."
- Preface to Romans by Martin Luther (1552), The Works of Martin Luther, Volume VI (pp. 447–462), Muhlenberg Press, Philadelphia
- A Commentary on St. Paul's Epistle to the Galatians, by Martin Luther (ntslibrary.com) pdf. "In order to have faith you must paint a true portrait of Christ. The scholastics caricature Christ into a judge and tormentor. But Christ is no law giver. He is the Lifegiver. He is the Forgiver of sins. You must believe that Christ might have atoned for the sins of the world with one single drop of His blood. Instead, He shed His blood abundantly in order that He might give abundant satisfaction for our sins."
- The Institutes of the Christian Religion by John Calvin (ntslibrary.com) pdf
- The Fire of Purgatory Comes from Hell, Laramie Hirsch Laramie, OnePeterFive, October 30, 2018.
- 1 John 3:5; 4:13; 5:20. If in him is no sin, and the sin or blot remains in Christians covered over and never removed, then Christians can never be in him, because "in him is no sin". But according to these verses in 1 John Christians are "in him", and therefore no sin remains in them, because they are in him, and in him is no sin, because he has taken their sins away.
- The Atonement of Jesus Christ, Elder Jeffrey R. Holland (jesuschrist.lds.org, Accessed March 2, 2008). A Mormon view of the Atonement, as relayed by an LDS Elder.
- Martin Luther's Bible Commentary - Romans and Galatians (biblestudytools.com, Accessed 29 September 2017)
- The Doctrine of Atonement - Catholic Encyclopedia (newadvent.org, Accessed 29 September 2017)
- Penal Substitutionary Atonement - Theopedia (theopedia.com) A discussion of differing views of the 16th century Protestant theological innovation of atonement by substitution.
- Christ's Atonement as the Model for Civil Justice, Jeffrey Tuomala, Liberty University School of Law (digitalcommons.liberty.edu, Accessed 29 September 2017)