Atonement means to make reparations for a wrongdoing, and for us, that wrongdoing is sin. When Adam and Eve were in the Garden of Eden, God gave them two choices (Gensis 2:16-17).
1. Don’t eat the fruit from the tree that cointains the knowledge of good and evil (correct)
2. Do eat the fruit (wrong - cue the buzzer noise)
God had told them they could eat the fruit from any tree but that one, and if they did it eat, they would die. Adam and Eve chose to eat the fruit when Satan tempted them with it, and they brought sin and death into the world (Genesis 3:1-19).
Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned.” Romans 5:12, NIV
When they ate the fruit, they suddenly realized they had been naked this entire time! They were ashamed of this and made clothes out of fig leaves, but God covered them with animal skins. Nothing Adam and Eve did could cover their guilt and shame - only the shedding of blood could atone for their sin (Genesis 3:7, 3:21).
Enter the Mosaic Law. God gave Moses a lot of instructions on how the Israelites were to interact with Him. There was a portable temple for God’s presence to dwell among them (check out this post for more info on that), priests to go between the people and God, and lots of sacrifices.
Once a year, on the Day of Atonement, the High Priest would make a sin offering for the entire nation of Israel. There’s a lot of details in the whole procedure, but the main thing is this: several critters had to die.
For the life of a creature is in the blood, and I have given it to you to make atonement for yourselves on the altar; it is the blood that makes atonement for one’s life.
Leviticus 17:11, NIV
Like with Adam and Eve, the only acceptable payment for sin was blood. Basically, a life for a life.
Finally, we get to Jesus. At just the right time, God sent His Son into the world to make atonement for our sins (Romans 5:6).
The book of Hebrews tells us that the blood Jesus shed to forgive our sins was perfect. We don’t have to sacrifice any more goats!
For Christ did not enter into a holy place made with human hands, which was only a copy of the true one in heaven. He entered into heaven itself to appear now before God on our behalf. And he did not enter heaven to offer himself again and again, like the high priest here on earth who enters the Most Holy Place year after year with the blood of an animal. If that had been necessary, Christ would have had to die again and again, ever since the world began. But now, once for all time, he has appeared at the end of the age to remove sin by his own death as a sacrifice.
Hebrews 9:24-26, NLT
He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world.
1 John 2:2, NIV
Jesus was a perfect sacrifice. His blood doesn’t just cover sins, like the blood of animals, it removes sins. Unlike the Day of Atonement, which occurred every year, Jesus only had to die once.
The reparations Jesus made for sin is good for all time, forgives all sin, and is available to all people. You don’t have to try and cover your sin like Adam and Eve did. You can just accept the payment Jesus made on your behalf.