In the Old Testament, the Israelites were given many commandments and then told what to do if they failed to carry them out.  Leviticus, chapter 5, gives some examples.  One of these passages reads,  "When anyone is guilty in any of these ways, he must confess what he has sinned and, as a penalty for the sin he has committed, he must bring to the LORD a female lamb or goat… as a sin offering; and the priest shall make atonement for him for his sin."  (Lev. 5:5, 6, NIV – italics added)  A poorer person was allowed to bring two young doves or pigeons (verse 7), but no matter which animal was brought, as Hebrews 9:22 tells us, "without the shedding of blood there is no forgiveness." (NIV) 

Atonement was a payment for the sin that made the sinner "right" with God again.  We also read in Hebrews that the sacrifices under the Law were "types" (pictures) of the great sacrifice of Jesus Christ, which was to come.  "It is impossible for the blood of bulls and goats to take away sins.  Therefore, when Christ came into the world, he said:  '….I have come to do your will, O God.' ….And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all."  (Heb. 10:5-10, NIV)

There is only one scripture in the New Testament which uses the word, "atonement":  "And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement."  This is Romans 5:11, King James Version.  Many more modern versions of the Bible, including the NIV, the NASB, and the NWT use the word "reconciliation" instead of "atonement."  

The word for "atonement" or "reconciliation" in Romans 5:11 comes from the Greek word katallassoAccording to Vine's Expository Dictionary, katallasso "properly denotes… to change from enmity to friendship, to reconcile."   When Adam and Eve were created perfect, they had perfect communion with God.  They could have lived for ever if they had not sinned.  After they sinned, they lost this perfect communion and their right to perfect life on earth.  There was need for an atonement between God and mankind, a reconciliation.  God is love, but He is also just.  God needed a just way to bring back the human race to Himself and to perfect life.

The human race had been lost through Adam.  If there could be another perfect man who would die in place of Adam, then justice could be satisfied, and mankind could be restored.  So "when the fullness of the time came, God sent forth His Son, born of a woman…"  (Gal. 4:4, NASB)  Jesus was "holy, innocent, undefiled, separated from sinners" (Heb. 7:26 NASB), a corresponding price for perfect Adam.  There was, and is, no price we can pay, no sacrifice we can make, to atone for our sins.  But Jesus has paid the price; and if we accept Him, we are reconciled to our Father in heaven.

There is another word in the New Testament which means "atonement."  It is "propitiation."  "…If anyone sins, we have an Advocate with the Father, Jesus Christ the righteous; and He Himself is the propitiation for our sins; and not for ours only, but also for those of the whole world."  (I John 2:1,2 NASB)  Jesus bought the whole world of mankind, and His ransom sacrifice will apply to all people, "in due time."  (I Tim. 2: 5,6)