Atonement: means to cover. In the old testament it is related to God's forgiveness.

The Hebrews were to bring an offering to the priest, who was responsible for making atonement for them, before Jehovah. Leviticus 6: 7 "and the priest will make atonement for him before the Lord, and he will obtain forgiveness for whatever he usually offends."
The sins of those who lived under the Levitical system were forgiven, they continued to carry the burden due to their sinful nature. Every time they sinned, they had an obligation to make an offering.
But sin still had power over them.
The apostle Paul said:
"As sin entered the world through one man, and through sin death passed to all men because all sinned."
The wages of sin is death but Jesus willingly paid the price for our sins and reconciled us to the Father. It is for this reason that Jesus exchanged our sinful nature for His righteousness so that we can have eternal fellowship with Him.
God did not forgive one by one but all at once, currently the atonement for sins does not exist, we obtain forgiveness by accepting Jesus as our Lord and Savior He is the lamb that takes away the sin of the world.