The New Testament makes it clear that Jesus Christ atoned for the sins of all who repent and acknowledge their part in Christ's death, a recognition that their sins caused Jesus to die.
One of the seven Holy Days of the Bible is also associated with atoning for sins. It's the Day of Atonement.
When in 1973 Egypt attacked Israel on the Day of Atonement, it became known as the Yom Kippur War (Yom Kippur is Hebrew for Day of Atonement). Up to the time of Jerusalem's destruction by the Romans in A.D. 70, the high priest would seek forgiveness for the sins of the whole Jewish community, and by so doing would reconcile his people to God.
For a Christian, one focus on this Day of Atonement is the removal of the primary cause of sin—Satan and his demons. As quoted in our booklet, God's Holy Day Plan: The Promise of Hope for All Mankind,"Until God removes the original instigator of sins, mankind will simply continue to fall back into disobedience and suffering. Although our human nature has a part to play in our sins, Satan the devil bears great responsibility for influencing mankind to disobey God." The booklet goes further and explains the fate of Satan at the end-time when God intervenes in the affairs of man.
Satan is the main source of evil in the world
I remember only too well an incident that occurred at work earlier in my life. A work colleague and I were discussing the existence of God. I was shocked when he expressed to me that, although he believed in God, he believed that God was evil. I asked him how he could say such a thing. He answered that it was on account of all the terrible things, all the evil that happens in the world on a daily basis. My workmate had experienced war directly and saw enough evil being perpetuated in coming to this conclusion.
Although his view of God was shocking to me, I could not deny the fact that evil is perpetuated among the sons of men. Only later, after coming to an understanding of God's plan for mankind, which the above mentioned booklet explains, did I realize that my work associate had made one vital mistake in his otherwise astute observance about evil in the world.
It's not the Almighty who is to blame for the evil we see, but rather Satan himself, who is called in the New Testament "the god of this world" as the King James Bible translates it. The evil we see all around us must be laid primarily at his feet. The New King James refers to Satan as "the god of this age" (2 Corinthians 4:4 2 Corinthians 4:4In whom the god of this world has blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine to them.
American King James Version×). The apostle Paul refers to him in the following way when he encouraged his Christian readers: "in which you once walked according to the course of the world, according to the prince of the power of the air, the spirit who now works in the sons of disobedience" (Ephesians 2:2 Ephesians 2:2Wherein in time past you walked according to the course of this world, according to the prince of the power of the air, the spirit that now works in the children of disobedience:
American King James Version×).
Satan's evil influence to be removed by Jesus Christ
Satan has been allowed to influence many leaders in society—political as well as religious leaders—to follow a course of action that is in opposition to God's way of life. The experience of the patriarch Job in the Bible is a prime example of Satan being the cause of many of the evil events befalling the world. It was he who brought destruction to Job's family and his servants in the hope that Job would turn against God, just like he tried to have Jesus Christ turn against His Father (Matthew 4:1-4 Matthew 4:1-4  Then was Jesus led up of the spirit into the wilderness to be tempted of the devil.
 And when he had fasted forty days and forty nights, he was afterward an hungered.
 And when the tempter came to him, he said, If you be the Son of God, command that these stones be made bread.
 But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceeds out of the mouth of God.
American King James Version×). But both Job and Jesus continued to acknowledge God as the Almighty. Job stated, "Naked I came from my mother's womb, and naked shall I return there. The Lord gave and the Lord has taken away; blessed be the name of the Lord" (Job 1:21 Job 1:21And said, Naked came I out of my mother's womb, and naked shall I return thither: the LORD gave, and the LORD has taken away; blessed be the name of the LORD.
American King James Version×).
The meaning of the Day of Atonement shows that one of the first acts that Jesus Christ will perform once He returns to establish the Kingdom of God is to banish Satan to the "bottomless pit" so that he can deceive the world no longer. That is exactly what he has done for the entirety of man's existence—to deceive and lead people astray from worshiping the true God. But at Christ's return he is to "deceive the nations no more" (Revelation 20:1-3 Revelation 20:1-3  And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand.
 And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years,
 And cast him into the bottomless pit, and shut him up, and set a seal on him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.
American King James Version×).
The Day of Atonement is given to us in God's Word so that we may humbly reflect on all the evil we see in the world for which Satan is primarily responsible. All the suffering that the world has experienced over the millennia will ultimately be brought to an end after God's Kingdom has been established on the earth. This Holy Day has great significance for the entire world, and God's true Church is keeping it, awaiting the time when Satan will "deceive the nations no more."
We recommend the previously mentioned booklet, God's Holy Day Plan: The Promise of Hope for All Mankind. To read more about the Day of Atonement, read this corresponding chapter - "Atonement: Removal of Sin's Cause and Reconciliation to God".