A fundamental belief of the Christian faith is that Jesus Christ, the Son of God, died on the cross for the sins of the world.

But why did Jesus have to die on a cross, as opposed to hypothetically living a long life and passing away of natural causes?

Throughout the history of Christianity, many theories and sub-theories on the doctrine of the atonement have manifested to explain the why of Jesus' death.

These atonement theories, sometimes called models or perspectives, often overlap and can be at times confusing to differentiate.

Adonis Vidu, associate professor of Theology at Gordon-Conwell Theological Seminary in South Hamilton, Massachusetts, told The Christian Post that these ideas should be considered part of "larger families of theories, rather than single theories."

"Each theory is trying to preserve important biblical motifs. The contemporary discussion isn't so much about which theory is the right one, but about how the various models of the atonement are to be logically related to one another," explained Vidu.

"We have moved beyond thinking that we have to make a choice of one theory. Almost each theory affirms important truths; the question is how all of these truths weave a coherent and biblically faithful tapestry of the work of Christ."

There are four major categories of atonement models that are generally agreed upon by most theologians.

1. Ransom Theory

Taking upon itself the image of Jesus being the "ransom for many," Ransom Theory holds that Jesus became a ransom for sinners held captive by Satan.

In so doing, Jesus paid the price owed to Satan for humanity, which before the crucifixion of Christ was held in slavery to the Devil.

A variation of Ransom Theory is the "Christus Victor" Theory, which appeared in the 1930s and stresses God's victory over sin and death through the crucifixion.

Feminist Theologian Katie M. Deaver of the Lutheran School of Theology at Chicago explained to CP that Ransom and Christus Victor are sometimes considered separate theories.

"While the Ransom and Christus Victor theories, technically, are two separate theories academics often consider them together because both are understood to be attempts of the early Church Fathers to explain the atonement event," explained Deaver to CP.

"In many cases research in this area will focus on the three primary theories of the atonement with the Ransom and Christus Victor theories becoming one combined understanding of the atonement."

Deaver explained that the atonement theory she most identified with was Christus Victor, with her saying that the model "takes into account Christ's incarnation, life, ministry, death, and resurrection rather than focusing primarily on the death/passion narratives."

"By emphasizing all of these aspects of the person of Christ, the conflict against evil becomes a continuing battle against all forces of evil which God continues to undertake for all of humanity," said Deaver.

Glenn R. Kreider, professor of Theological Studies at Dallas Theological Seminary, told CP that the Ransom Theory had some biblical justification, yet also held some shortcomings.

"Of substitutionary theories, the ransom theory has the benefit of explicit biblical support. Matthew 20:28 and Mark 10:45 assert that Jesus gave his life as a ransom," said Kreider.

"But throughout church history, there have been multiple attempts to answer the question, to whom was the ransom paid? The Bible does not address it, focusing instead on the costly price of our redemption, the life of the Son of God. This theory is helpful but inadequate by itself."

Ligonier Ministries has offered extensive criticism of the Ransom Theory, having stated on their website that there are "several problems with this view."

"First of all, the Bible does not speak of us owing Satan anything. Secondly, it gives Satan much more power and credit than he deserves. Finally, it neglects the biblical idea that any debt we owe is a debt owed to God," stated Ligonier.

"Sinners are in bondage to Satan. But they are only in bondage because God has allowed them to be. The bondage mankind finds itself in is part of the punishment for sin. Payment is owed to God, not the Devil."

2. Substitution Theory

Also called Penal Substitution, the theory and its variations emphasize Christ's suffering on the cross as a substitute for sinful humanity.

"This theory sees the atonement of Christ as being a vicarious, substitutionary sacrifice that satisfied the demands of God's justice upon sin," explained the Christian site gotquestions.org.

"With His sacrifice, Christ paid the penalty of man's sin, bringing forgiveness, imputing righteousness, and reconciling man to God."

Drawing its heritage to the 1500s and the Protestant Reformation, Substitution is a very popular atonement theory, with one critic on Patheos writing that, thanks to Reformers like John Calvin, Penal Substitution "has become the default position."

Kreider of Dallas Theological Seminary said of the atonement models, he preferred the Substitution view, saying that it is the only one that "is distinctly Christian."

"There are a variety of substitutionary models. What they have in common is that Jesus, the innocent one, died instead of the guilty. From the beginning of the story of redemption, substitution is emphasized," he explained.

"The language of the Nicene Creed asserts that it was for us and our salvation that Jesus came down from heaven and that it was for our sake that he was crucified. That is language of substitution."

Kreider noted scriptural examples going back to Genesis, "when Adam and Eve sinned, God provided clothes of animal skin."

"An animal died to atone for their sins. In the sacrificial system, atonement was accomplished through the death of an animal. Jesus is the Lamb of God who takes away the sin of the world (John 1:29); he fulfills the typology of the Old Covenant sacrificial system," said Kreider.

"Jesus voluntarily laid down his life for us (John 10; Matt. 20:28). He died for us, instead of us, as our substitute, while we were yet sinners (Rom. 5:8). God graciously provided salvation for us through his Son by grace (Eph. 2). The wages of sin is death but life is a gift of God's grace (Rom. 6:23). Jesus received our wages instead of us."

Robert C. Crouse, assistant professor of Systematic Theology at Knox Seminary, located in Fort Lauderdale, Florida, told CP that Penal Substitution was "the most central and the foundation for all the other points of view."

3. Moral Example Theory

Grouped with and also known as Example Theory, Exemplary Theory, and Moral Influence Theory, this model focuses more on the life of Jesus rather than the crucifixion or resurrection.

"This view sees the atonement of Christ as simply providing an example of faith and obedience to inspire man to be obedient to God," noted gotquestions.org.

"Those who hold this view believe that man is spiritually alive and that Christ's life and atonement were simply an example of true faith and obedience and should serve as inspiration to men to live a similar life of faith and obedience."

Moral Example has been criticized for deemphasizing the importance of the divinity of Jesus, as well as the need for sins to be forgiven.

Crouse of Knox Seminary told CP that while there was biblical justification for all the major atonement models, he found this model to have notable shortcomings.

"I think the Exemplary model is there in Scripture — we should follow Christ's example on the cross of self-sacrificial love — but as a perspective of God redeems sinful humanity, it is very limited," argued Crouse.

"In other words, the Exemplary model 'works out' of Penal Substitution (and others), not 'toward' it. Once Christ has objectively removed the legal demands, we can be freed to follow his way of discipleship."

Deaver of the Lutheran School of Theology at Chicago spoke positively of the Moral Influence Theory, believing that "understanding Christ as a moral example can certainly be helpful for everyday Christians."

"I believe that these atonement theories become incompatible with sound Christian doctrine when we allow them to be interpreted or taught in ways that are oppressive and harmful," noted Deaver.

"For example, these theories of atonement have often been used to encourage women to remain within abusive relationships because it is 'their cross to bear' or they ought to live by Jesus' example and suffer just as he did. In cases like this the real problem lies not with the theories themselves but in the ways that we teach those theories to people of faith."

4. Satisfaction Theory

Seen as a forerunner of Penal Substitution Theory, Satisfaction Theory was developed by the 11th century Archbishop of Canterbury, Anselm.

"Anselm argued that it was necessary for the atonement to take place in order to satisfy the justice of God," explained R.C. Sproul of Ligonier Ministries in 2013.

"That viewpoint became the centerpiece of classical Christian orthodoxy in the Middle Ages, in terms of the church's understanding of the work of Christ in His atonement."

Anselm's work remains prominent within the Roman Catholic Church, with Catholic Encyclopedia noting that "few pages of our theology that have not been illustrated by the labours of Anselm."

"His treatise on the procession of the Holy Spirit has helped to guide scholastic speculations on the Trinity, his 'Cur Deus Homo' throws a flood of light on the theology of the Atonement, and one of his works anticipates much of the later controversies on Free Will and Predestination," noted the Encyclopedia.

This article was first published in The Christian Post.