The term atonement in Christian theology refers to the Triune God‘s acting in human history to reconcile the original relationship between God and man that was broken due to sin. Biblically, to atone means to make amends and reconcile whereby the guilt of man is removed. In the Old Testament, atonements were offered by the high priest but were only temporary and had to be offered repeatedly. They foreshadowed the real and final atonement made by Jesus Christ on the cross where He atoned for the sins of the world (1 John 2:2) and, when one receives and accepts this atonement by faith (Ephesians 2:8-9), they are reconciled to God (Romans 5:9-11) and have peace with Him (Romans 5:1).
The atonement was completed on the cross, where the blood of Christ was shed (Hebrews 9:22) and where sin was “put away” entirely (Hebrews 9:26). This is why part of the promise of the New Covenant is that God remembers believers sins no more (Jeremiah 31:31-4). This is much different than the Adventist teaching.
Man is a sinner (Romans 3:23; 5:8) and cannot atone for himself. Therefore, God the Father, in His love for humanity sent His Son (1 John 4:10) to die in our place (1 Pet. 3:18) for our sins (1 Pet. 2:24). It is because of the atonement that our fellowship with God is restored.