In the beginning when God created man and placed man in the Garden of Eden, they had a close relationship. God visited them in the Garden from time to time and there was Joy and Peace. But man, disobeyed God and that relationship was broken.
The first sin man committed was disobedience. But before then, God told them something in Genisis 2:16&17, that "if you eat of this tree, you shall surely die". So that was what happened, they died. You might be thinking, why didn't they die there? Or why didn't they fall down immediately as corpses? They died spiritually, the Spirit of God left them, and so the flesh (Self-life) was replaced in them. Romans 6:23 clearly states the penalty for Sin.
God because of his Love for the world (John 3:16), sent His only begotten Son Jesus Christ, to die for us. He took upon Him our sins and died for us, upon Him He took our punishment and chastisement for sin. But thanks be to God who borne our iniquity.
In the old days before Jesus Christ died for the world, they sacrificed sheep, goat, etc. To cover their sins. So, imagine how many times you sin and how many animals you slaughter just to cover your sins. Because of Jesus Christ's death, we need not to sacrifice animals anymore. Hebrews 10:14 says, "so that we will be sacrificed through the offering of the body of Christ once and for all".
In the old days the highest punishment for sin is to be hanged on a cross. In the Jewish tradition anyone who is hanged on the cross is cursed. But thanks be to God, Jesus Christ was made curse so that we will be delivered from curse. Galatians 3:13 enlightens that.
Jesus Christ himself was not guilty of any sin, He had done no violence, and no deceit on His mouth. But the Bible says, "the Lord had laid upon Him the iniquity of the world". Christ Jesus didn't only identify our iniquity, but He also endured all the evil consequences of our sins. He carried them away so that they might never return again upon us.
Isaiah 53:4-6("He borne our griefs and carried our sorrows"). Even though the verbs are past tense, they predict the future to the old time. Isaiah was saying that the Messiah would bear the consequences of the sins of men, namely the sorrows of life, though incredibly the Jews watched Him die thinking He was being punished by God for His own sins. Matthew found an analogical fulfillment of these words in Jesus Christ's healing ministry (Matthew 8:16&17; 1Peter 2:24). In eternity, all sickness will be removed, so ultimately it is included in the benefits of atonement.
Jesus Christ suffered not for his own sin, since He was sinless (Hebrews4:15; Hebrews7:26), but as the substitute for sinners. He was substituted for God's wrath for sinners (Hebres10:9&10). Every person has sinned (Romans 3:9&23), but Jesus Christ has sufficiently shouldered the consequences of sin and the righteous wrath deserved by sinners (1John 2:2)
The manner in which God laid our iniquity on Him was that God treated Him as if he had committed every sin ever committed by every person who would ever believe, though He was perfectly innocent of any sin. God did so to Him, so that wrath being spent, and justice satisfied, God could then give to the account of Jesus Christ, treating them as if they had done only the righteous acts of Christ.
Jesus Christ's righteousness was imputed on us to make us righteous.