David L. Allen, The Atonement: A Biblical, Theological, and Historical Study of the Cross of Christ (Nashville: B&H Academic, 2019), 15-16.
The English word “atonement,” first used in 1526 by William Tyndale in his English translation of the NT, renders the Greek word katallagē (“reconciliation”) in Rom 5:11. However, the word “atonement” itself does not correspond etymologically to any Hebrew or Greek word. This English word expresses the concept of “at-one-ment” (i.e., reconciliation) when the benefit of the work of Christ is applied to one who believes.
In the NKJV translation, the word “atonement” appears ninety-seven times, exclusively in the OT. In the CSB and ESV translations, the word is used eighty times in the OT and twice in the NT. Acts 27:9 refers to the “Day of Atonement.” In Heb 2:17, “atonement” is used to translate
the Greek word hilasmos, which connotes both propitiation and expiation of sin by the work of Christ on the cross. The word indicates objective reconciliation with all humanity in the sense that the removal of all legal barriers between sinful humanity and God renders humanity to be “savable.”
What is meant by the phrase “removal of legal barriers” (as used by theologians of the past, both Calvinist [such as A. A. Hodge, W. G. T Shedd, and James P. Boyce] and non-Calvinist, and as I am using it now)? Removal of legal barriers in the atonement of Christ is not tantamount to justification, such that there is no legal basis for condemnation of a person due to his sin. Atonement and justification are two distinct things. God cannot save people simply by an act of His will (voluntarism). The righteous requirement of the law must be satisfied in order for God to approach humanity with offers of mercy. In the cross God has taken away that legal necessity, thereby providing a righteous path for forgiveness. He has removed all things on His part that stood in the way of His being able to offer forgiveness in a just way (Rom 3:21–26).
The great theologian James Denney understood the concept well:
The work of reconciliation, in the sense of the New Testament, is
a work which is finished . . . before the gospel is preached. . . . It is
a work outside of us, in which God so deals in Christ with the sin
of the world that it shall no longer be a barrier between Himself
and man. . . . Reconciliation is not something which is doing; it is
something which is done.
Likewise, James Pendleton argues,
So far as the claims of law and justice are concerned, the atonement
has obviated every difficulty in the way of any sinner’s salvation.
In supplying a basis for the exercise of mercy in one instance
it supplies a basis for the exercise of mercy in innumerable instances.
It places the world, to use the language of Robert Hall, “in
a salvable state.” . . .
Without the atonement, there can be no salvation. Because of the atonement, all humanity is rendered “saveable.” The effectual application of the atonement by the Holy Spirit to the sinner who believes in Christ results in salvation.
 James Denney, The Death of Christ: Its Place and Interpretation in the New Testament (London: Hodder & Stoughton, 1912), 145–46.
 James M. Pendleton, Christian Doctrines: A Compendium of Theology (1878; repr., Valley Forge, PA: Judson Press, 2010), 242.