Atonement is what it costs to take something that is wrong and put it right.
We are wrong. How did I come to that conclusion? Well, sin is wrong. Sin fills the earth and is in the very nature of humanity. We are born into that sinful nature of humanity. Therefore, we are "wrong" and need atonement to make the wrongs right so that we can be at peace with the creator of the universe who doesn't have a "wrong" bone in his body (so to speak).
Hebrews chapter 2 talks all about this.
"How shall we ignore so great a salvation?" (Heb. 2:3)
How many of us ignore, or chose to disregard the atonement Jesus made for our sin?
"What is mankind that You are mindful of them?" (Heb. 2:6)
We certainly didn't deserve that atonement.
Here's the kicker:
"Both the one who makes people holy and those who are made holy are of the same family. So Jesus is not ashamed to call them brothers and sisters." (Heb. 2:11)
Jesus calls us his family! He deems us worthy to be his brothers and sisters! The reason he is not ashamed to call sinners like ME his family is because of what he did for me on the cross when he TOOK AWAY MY SIN and made ATONEMENT for me before God! He did this so that the chasm separating me and my sinful nature from God and all his love and holiness has been filled! He did this so that I can now be called a Child of God! How incredible!
"Since children have flesh and blood, he too shared in their humanity so that by his death he might break the power of him who holds the power of death - that is, the devil - and free those who all their lives were held in slavery by their fear of death." (Heb. 2:14-15)
There are so many levels and effects of what Jesus has done. He SAVED us from our sin, served as our ATONEMENT once and for all, freed us from the bondage of slavery that is sin, and he took away the most basic human fear, the fear of death. Death is no longer a devastating end, but a glorious reuniting of us and our Savior and Creator! He is ready and waiting to welcome us.