Salvation, in its true and full meaning, is synonymous with exaltation or eternal life and consists in getting an inheritance in the highest of the three heavens within the celestial kingdom. With few exceptions this is the salvation of which the scriptures speak. This resolution also does not concern whether or not it is possible to lose one's salvation. The question is, how is salvation accomplished, not how salvation is lost. Salvation is being rescued from the deserved wrath of God.
Man is bound by sin.
Atonement means the reconciliation of the guilty to God by divine sacrifice, that of Jesus Christ on the cross. Because Christ's atonement accomplishes all this, there is nothing left for us to do. All of our sins are forgiven, atoned for, and covered by Christ's one sacrifice on the cross. No human effort can add to Christ's all-sufficiency. He did once for all when He offered up Himself.