The word (at-one-ment) is used but once in the New Testament (Rom. 5:11), and there it means expressly and exactly reconciliation. This is proved thus: the same Greek word in the next verse, carrying the very same meaning, is translated reconciliation. Now, people continually mix two ideas when they say atonement: One is, that of the expiation for guilt provided in Christ's sacrifice. The other is, the individual reconciliation of a believer with his God, grounded on that sacrifice made by Christ once for all, but actually effectuated only when the sinner believes and by faith. The last is the true meaning of atonement, and in that sense every, atonement (at-one-ment), reconciliation, must be individual, particular, and limited to this sinner who now believes. There have already been just as many atonements as there are true believers in heaven and earth, each one individual.
We have grown accustomed to using the word "atonement" for the work of Christ's satisfaction apart from the application. I think that's an error, but not necessarily a great one. Good men (Shedd, for example) have used that term inaccurately. I believe the bible reserves the idea of "atonement" for the point of application. In other words, when one has truly trusted Christ, his sins are "atoned" and not before. He is then at one with Christ through faith... Atonement is a word that should be reserved for the point of the efficacious application (or real union), and not for Christ's work on the cross intrinsically considered (when we are merely in virtual union with him). This virtual/actual union distinction is a crucial point that frequently gets confused or blurred in arguments.