Jesus Christ was not going to substitute us for our sin. We need to face the destiny of our sins! However, according to the principle of Law of Justice, any one who sins is sin against the law given him, and also sin against the law giver, because of the principle, anyone who does unlawfully to the other does actually sin against the law giver first, and because it is the God who is the law giver and the steward of the whole universe, anyone can not do anything unlawful to anyone unless he sin against the God first and permitted according to HIS wisdom that the sinner must pay and the victim must retrieve the loss. The sinner must pay for what he does! He has to pay to the God because everything is given by HIM including his life, so the ultimate penalty is loss of life.
Jesus Christ was the God and the lawgiver, so anyone who sins against Him has to pay for it according to the Law of Justice, unless He forgives him. He came to this world to forebear the sins against him by the sinners up to his death on the cross so that the world was judged and sinner was destined to eternal damnation!!! Fortunately our God and lawgiver is kind and full of grace. He is willing to forgive us if we repent for our sins. Most of out sins will be forgiven under the condition of repentance, but those sin against Holy Spirit and involving living rights of other cannot be forgiven, the sinner has to ask for the forgiveness of the real victim also. Deprivation of lives or chastity of others is considered unpardonable crimes in God.

The real meaning of Atonement of Sin ?
Atonement in greek is hilaskomai which come from root hila i.e. merciful. Sin in greek is hamartia which means out of the road or astray. An atonement is really a kind of merciful to the one who go astray.
II Corinthians 5:18-19 18And all things are of God, who hath reconciled us to himself by Jesus Christ, and hath given to us the ministry of reconciliation; 19To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation
Atonement is the mercy of God to people who tresapass the law and astray from HIM, through the words of reconciliation brought by Jesus Christ, who is from HIM.