sacrifices, which can never take away sins. But when Christ had offered for all time a single sacrifice for sins… by a single offering he has perfected for all time those who are being sanctified.” (Hebrews 10:11-14)
So in summary, atonement is a bringing back together between mankind and God. To do this, sin has to be removed first because God cannot associate with sin. And it took the perfect sacrifice of the Lord Jesus Christ to achieve this for all time – before that, it was just a temporary solution. Because of Jesus Christ, if we confess our sins to God, and commit our lives to Him, they really will be forgiven.