One of the more difficult things to understand in the Bible is exactly how Jesus' death on a cross pays for all of humanity's sins. There are many different theories on exactly why Jesus had to die, and I suspect that you can probably understand it from a variety of viewpoints.
Why does it matter?
It is true that we can just take it on faith that Jesus' death pays for our sins. The Bible makes this clear, and I don't believe it is absolutely necessary to know the theory behind it. The Christian author C.S Lewis wrote "The central Christian belief is that Christ's death somehow put us right with God and given us a fresh start. Theories as to how it did this are another matter."[1 p.54]. C.S Lewis also compares it to how when a person is hungry, they don't have to know the biology of nutrition and digestion in order to eat and feel full. When we get to heaven we will not be quizzed on exactly how Jesus death paid for our sins - we only need to have faith that it did pay for our sins.
However, the study of the theories of atonement can be useful. It can boost the faith of people whose faith is weak, and it is good to know to explain why Jesus had to die to non-Christians. So lets get into what the theories are.
Alternate Theories of Atonement
"Atonement" in Christianity means "The reconciliation of God and humans brought about by the redemptive life and death of Jesus.". Throughout church history there have been many different theories of how atonement happened, and some denominations prefer certain explanations. A good summary can be found here . I will not go into all of that detail, because I feel that it does not matter who said it or who likes it or how long it has been popular - it only matters what the Bible says. Many of these theories don't seem to have actual Biblical support to back them up, and so I will focus on the few that might actually match the Bible.
This theory is that Jesus' death was a ransom payment to set humanity free. This is based on Mark 10:45 "For even the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many." (NIV version). But the question is who is the ransom paid to? As discussed here , often it was believed that when Adam sinned, it set all humanity as captives to Satan, and God had to send Jesus to die as payment for Satan to let humanity go. But then because God loved Jesus, he resurrected Jesus afterwards. According to , this theory was the earliest theory of atonement and was popular up to 1100 A.D. I agree with the author of  who states that this theory doesn't completely work, because there is no proof in the Bible that Satan holds humanity hostage and demanded Jesus to die as a ransom. Also, why then would Satan only release the ones who accept Jesus death, if Jesus death was the ransom for all humanity? It doesn't totally work.
The Contemporary English Version translates Mark 10:45 as "The Son of Man did not come to be a slave master, but a slave who will give his life to rescue many people.". So this does not necessary imply a "ransom", but states that the act of Jesus dying rescued us from something. This something does not have to be Satan, but could be rescuing us from our fate of going to hell, or rescuing us from God's punishment, etc. If it is translated this way, this verse could match with the more common Substitution/Sacrifice theory as well.
Christus Victor means "Christ is the victor" and claims that Jesus' death conquered evil/Satan/death, etc. and now we are no longer bound by it. This has some scriptural support, such as:
- "As surely as we died with Christ, we believe we will also live with him. We know that death no longer has any power over Christ. He died and was raised to life, never again to die." (Romans 6:8-9)
- "I tell you for certain that anyone who sins is a slave of sin!...If the Son gives you freedom, you are free!" (John 8:34)
- "You died with Christ. Now the forces of the universe don't have any power over you." (Colossians 2:20)
- "In the past you were dead because you sinned and fought against God. You followed the ways of this world and obeyed the devil. He rules the world, and his spirit has power over everyone who doesn't obey God" (Ephesians 2:1-2)
- "But God set you free when he sent his own Son to be like us sinners and to be a sacrifice for our sin." (Romans 8:3)
- "God wiped out the charges that were against us for disobeying the Law of Moses. He took them away and nailed them to the cross. There Christ defeated all powers and forces. He let the whole world see them being led away as prisoners when he celebrated his victory." (Colossians 2:14-15)
Christ did conquer sin (as Jesus never sinned) and did conquer death (because of his Resurrection by God), did conquer hell (by creating a way for people to be saved), and indirectly conquered Satan (to be completed at the end of the Tribulation). Christians who believe in Christ also conquer sin (we are no longer condemned because of sin), and we can resist Satan (James 4:7), and we will be resurrected either at the Rapture or at the final Judgement. However, I don't think this theory explains how Jesus' death removes our sins - instead it explains more of the after-effects of accepting Jesus' payment for sin. Therefore, while we do have "more than a victory" in Christ (Romans 8:37), this is only after accepting Jesus' sacrifice for our sins, and the victory itself is not what removes sin.
This theory says "Sin is viewed as the withholding of honor due to God. In his death, since he was under no obligation to die, being sinless, Christ brought infinite glory to God. This brought a reward to Christ that he did not need so he passes it on to sinners if they live according to the gospel.". And also "Human sin dishonors God. A price must be paid to satisfy God and restore his divine honor. The only penalty suitable to God was Christ's obedience when he willingly suffered torture and death at his crucifixion.". This theory was based on medieval ideas of honour due to a lord, and if one offended the lord they would have to pay to "satisfy" the lord's honour.
There is some Biblical evidence that sin does dishonor God. "You take pride in the Law, but you disobey the Law and bring shame to God." (Romans 2:23). God is shamed/dishonored by sin, but it is the sin that has to be paid for, not God's dishonor: "How often have we sinned and turned against you, the Lord God? Our sins condemn us! We have done wrong." (Isaiah 59:12). There is no evidence that Jesus' death was paying for God's honour instead of our sins, and the Bible says much more about how bad sin is than how bad dishonoring God is.
There are many other theories also, but I cannot find much support for these in the Bible, and so I did not mention them here. The ones that have a little support from the Bible seem to still be incomplete, but may add additionally depth and meaning to Jesus' death. However, I feel the best theory which matches with the majority of Bible verses is the Substitution/Sacrifice theory which is explained below.
Substitution / Sacrifice Theory
This theory is the one most commonly found in Christianity, and I believe it has the most scriptural support. Therefore I will spend more time on it than the others. This theory says that since the penalty for sin is death (Romans 5:12 and 6:23), then if God loves humanity so much that he wants to forgive us (John 3:16), that God decided to pay our penalty for us. It makes Jesus guilty for all of our sins, who then suffered the consequences by dying on the cross. Then since God loves Jesus (and to prove Jesus really was God) God resurrected him. Now we can get into heaven if we accept Jesus' death as payment for our sins through faith. Here are some quotes from the Bible which back up this view:
- "God sent Christ to be our sacrifice. Christ offered his life's blood, so that by faith in him we could come to God" (Romans 3:25)
- "He was wounded and crushed because of our sins; by taking our punishment, he made us completely well." (Isaiah 53:5)
- "We had each gone our own way, but the Lord gave him the punishment we deserved." (Isaiah 53:6)
- "The Lord decided his servant would suffer as a sacrifice to take away the sin and guilt of others." (Isaiah 53:10)
- "Now that God has accepted us because Christ sacrificed his life's blood, we will also be kept safe from God's anger." (Romans 5:9)
- "Christ never sinned, but God treated him as a sinner so that Christ could make us acceptable to God" (2 Corinthians 5:21)
- "But Christ rescued us from the Law's curse, when he became a curse in our place." (Galatians 3:13)
- Jesus said "I am the good shepherd, and the good shepherd gives up his life for his sheep." (John 10:14)
- "Christ died once for our sins. An innocent person died for those who are guilty. Christ did this to bring you to God, when his body was put to death and his spirit was made alive." (1 Peter 3:18)
Additionally, this view works well when we consider that God told the Israelites to sacrifice animals to pay for their sins. However, animals cannot completely pay for a human's sin, and the sacrifices had to be done constantly. Jesus is called the "Lamb of God who takes away the sins of the world" (John 1:29). In this view, Jesus' death was the perfect sacrifice for all sin, once and for all (Romans 6:10). The animal sacrifices are no longer necessary, and they were really just a foreshadowing of Jesus' death for our sins (see my article here). I don't believe that God would have set up such a complicated sacrificial system if it was misleading about how atonement works. This sacrificial theory of atonement ties the Old Testament laws and the New Testament together, and should have convinced the Jews that Jesus was their Messiah since they were already familiar with the sacrificial system.
There is a different perspective of the substitution argument written by pastor Timothy Keller in his book "The Reason for God"[7, p.199]. Keller explains that Jesus had to die so that God could forgive us. It was not an arbitrary decision by God that he needs payment to forgive sin, but is because of how forgiveness itself works. If one is wronged, one can choose to punish the person who wronged them (justice) or can choose to forgive (mercy). However, forgiveness is not free - the person must absorb the wrong into themselves by suffering the pain/loss/cost of the wrong. For example, if a vandal comes and ruins your house, either you get the vandal to pay to fix it, or you pay out of your own pocket. If you are mad at someone who insulted you, you must either take your anger out on that person, or absorb it yourself. Since the penalty for our sins is death, then God had to absorb the death we all deserve into Himself by having Jesus (who was God) willingly die on the cross. This is how God forgives our sins - we don't have to pay for it because Jesus already did.
How does Jesus temporary death pay for all humanity's sins?
Doesn't it seem unfair? Jesus only died for three days until he was resurrected. But if we reject him, then we "die" for all eternity in hell. How is Jesus' temporary death a fair exchange for the death of all humanity in hell forever that we deserve?
First, note that death is not just a physical process - it is better defined as spiritual separation (from our loved ones on earth, and from God forever if we go to hell). Second, God is infinite and everlasting, and Jesus and God are part of the same being. When Jesus was on the cross, it says he cried "My God, my God, why have you deserted me?" (Mark 15:34). It seems God and Jesus' eternal closeness and connectedness was temporarily removed during Jesus' death. A limited separation of Jesus and God (who are both eternal) makes up for the eternal separation of all limited beings (humans) and God (assuming that everyone would go to hell if Jesus didn't die for us). This is how God took the punishment (separation) that we deserved and absorbed it into himself, so now we do not have to endure the punishment of eternal separation from God in hell.
Couldn't God have found another way?
I believe that God would not have sent Jesus to die unless there was some other option. If it were possible, God would have answered Jesus' prayer in the Garden of Gethsemane the night before he was arrested. Jesus prayed "My Father, if it is possible, don't make me suffer by having me drink from this cup."(Luke 22:42). Jesus didn't want to die, but it must have been the only way God could forgive us or else He would have found another way. Not only does Jesus' death show how serious our sin is considered by God, but also shows how much God loves us that he would be willing to die on a cross just for us. I believe that if you were the only person who had ever lived, God would still have sent Jesus to die just for you. That's how special you are to God, and how much He loves you.
Why do people still go to Hell if Jesus paid for all our sins?
So why can't God just apply Jesus' sacrifice to everyone in the world? Then everyone could go to heaven. It says repeatedly in the Bible that Jesus' death only pays for those who have faith in Jesus. I believe this is because of free will. God will not force someone to accept his gift of salvation - we must ask God for it. If someone does not want anything to do with God, it would be wrong for God to force them to spend eternity with Him in heaven. God does want everyone to be saved (1 Timothy 2:4), but will not force them to believe because God values free will. Free will is what allows real love and a real relationship with God, and that was the entire point of creating humanity.
So after examining several theories of atonement it seems clear that there is one main theory that has the most Biblical support and also makes the most theological sense. The Substitution/Sacrificial theory ties together the Old Testament Jewish sacrificial laws and the New Testament teachings of Jesus and the disciples into one cohesive story of salvation. It explains that God forgives us by taking the punishment we deserve for our sins into himself, through the sacrificial death of Jesus on the cross.
It is true that we might never understand completely how Jesus' death exactly pays for sins. It might take thousands of years of debate and analysis in Heaven before we figure it out. And we will be always grateful and amazed at that Jesus would rather die for us than have us be forever separated from him in Hell. God will be praised forever because of what he did, and it will never get old or boring. But still it comes back to C.S Lewis statement - the exact theory does not really matter as long as we know Jesus death does pay for/cancel out/satisfy/conquer our sins and we can live forever in Heaven by just accepting it through faith.
 C.S Lewis (1952) - Mere Christianity
 www.thefreedictionary.com - Atonement
 www.theopedia.com - Atonement of Christ
 www.gospel-mysteries.net - The Atonement
 www.theopedia.com - Satisfaction theory of the Atonement
 www.religioustolerance.org - The Satisfaction Theory
 Timothy Keller (2008) - The Reason for God