Have you heard of the Jewish holy day Yom Kippur? This is translated as, "Day of Atonement." Or, Day of Expiation. What does this mean?
William Lane Craig, in his book The Atonement, quotes Jacob Milgrom's Leviticus commentary. He writes:
“For the complete annulment of the sin..., for the assurance of divine forgiveness (sālaḥ), sacrificial expiation (kippēr) is always required” (1991, p. 377). Kippēr in its most abstract sense thus comes to mean atone or expiate. “The meaning here is that the offerer is cleansed of his impurities/sins and becomes reconciled, ‘at one,’ with God” (Milgrom, Leviticus, p. 1083).
The text [Leviticus] repeatedly promises, “the priest shall make atonement on your behalf for the sin that you have committed, and you shall be forgiven” (Lev 4.35; cf. 4.20, 26, 31, etc.). The word translated “make atonement” (kippēr) has a range of meanings – to purge, to ransom, to expiate – but what is significant here is the result: the person’s sins are forgiven. The ritual sacrifice has removed his guilt.
(Craig, The Atonement, pp. 11-12)