Assuming that the statement refers to Christian faith, through the history of Christianity there have been Christians who have believed in a separate soul (e.g. Descartes) and others who have not (e.g. St. Matthew) but the crux of the issue is whether such a belief is required.  This begs the question “by who or what standard?”  Obviously, belief in a separate soul is not required by the Creeds; the Apostles’ Creed affirms “I believe in the resurrection of the body and life everlasting” which suggests that Christian faith requires not a dualist but an avowedly monist position.  Further, the Catechism of the Roman Catholic Church 1059 affirms that “The holy Roman Church firmly believes and confesses that on the Day of Judgment all men will appear in their own bodies before Christ’s tribunal to render an account of their own deeds”.  It is clear that no belief in a separate soul can be required by orthodox Christian faith, although I will argue that belief in a separable soul might make it easier to sustain faith in the face of life’s challenges and apparent inequities.

Christian faith promises salvation; union with God and restitution for the injustices apparent in this life.  Nevertheless, the New Testament is unclear about how this salvation will come about and whether the afterlife will entail bodily existence or be purely spiritual.  The Synoptic Gospels suggest an immanent eschatology; descriptions of heaven and hell are earth-like and seem to suggest that people will have resurrected bodies to experience reward or punishment much as we experience these in the coming Kingdom of God.  Matthew 25 (the Parable of the Sheep and the Goats) suggests that the evangelists expected that Jesus will soon return to judge the living and the dead and supports a physical understanding of hell ‘Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels” (Matthew 25:41)  Luke 16:23-24 also supports this view “So he called to him, ‘Father Abraham, have pity on me and send Lazarus to dip the tip of his finger in water and cool my tongue, because I am in agony in this fire.’ Other references to the final judgement are similar “And I heard a loud voice from the throne saying, “Look! God’s dwelling place is now among the people, and he will dwell with them. They will be his people, and God himself will be with them and be their God. ‘He will wipe every tear from their eyes. There will be no more death’ or mourning or crying or pain, for the old order of things has passed away.” (Revelation 21:3-4) and this vision of the end-times seems consistent with Old Testament references, such as those in Ezekiel and Isaiah.  Christian faith supported by references in the synoptic gospels would require no belief in either a separate or a separable soul, only a belief in physical resurrection. Nevertheless, belief in physical resurrection is difficult to sustain in the modern world.  There is a complete lack of supporting evidence and it is difficult to see how it could deliver the promised reward (or punishment) in a fair and just manner.  Surely, those who died hundreds or thousands of years before the final judgement would be at greater risk of their bodes having disintegrated.  Surely, those who died as infants, after losing limbs, in extreme old-age or whose bodies were destroyed utterly would seem less likely to get their just deserts.  While “all things are possible with God” (Matthew 19:26) and Christian faith requires a belief in God’s omnipotence, it is clear that this cannot extend to God doing the logically impossible, or else most theodicies would collapse and God could not also be all-good. While God resurrecting people out of nothing by reassembling them from dispersed dust into their ideal form may not be logically impossible, it comes close to being so in some cases.

It is obvious why many 21st Century Christians prefer to believe in an eternal life, reward or punishment which begins soon after each person’s death.  A belief in immediate reward and punishment would work either with dualism, belief in a separate soul, or with a belief in re-creation into a parallel dimension.

Immediate reward or punishment through dualism, in a purely spiritual sense, is superficially easier to reconcile with science and reason.  There have been many reports of Near Death Experiences which, if credible, would to support belief in disembodied existence immediately after death.  Pam Reynolds’ experience during standstill surgery in 1991 is often seen as one of the best documented cases. More recent research conducted by Dr Sam Parnia at the University of Southampton might suggest that the soul could continue after death without a body.  In addition, a spiritual interpretation of the afterlife would be more rationally defensible than physical resurrection.  It is easier to see how a soul could survive eternally; a risen body would still be physical and so subject to aging, sickness, disability and other associated limitations.  It is easier to see how a soul could come “face to face” with God, who is not normally seen to have a physical existence as human beings do. Further, parts of John’s gospel, the Johannine letters and Paul’s letters seem to support a more spiritual interpretation of eternal life. Verses such as “So we fix our eyes not on what is seen, but on what is unseen, since what is seen is temporary, but what is unseen is eternal.” (2 Corinthians 4:18) and “So then, if anyone is in Christ, he is a new creation; what is old has passed away – look, what is new has come!” (2 Corinthians 5:17) and “Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life.” (Galatians 6:8) seem obviously Platonic in their influence and are closest to dualism. However, although faith with a purely spiritual eschatology seems easier to reconcile with science and reason, it comes with significant problems and has been relatively rare through the history of Christianity. Belief in a separate soul – dualism – is difficult to defend in philosophical or scientific terms and suggests other beliefs and practices which are incompatible with Christian theology. Descartes argued for a Christian dualism, but struggled to provide a coherent account of why the soul would be enfleshed, how soul and body interact and how a disembodied soul could experience reward or punishment in the way that would be necessary for Christian promises of eternal life to be meaningful.  The Catholic Church never accepted the idea that eternal life could be purely spiritual and disembodied because this might seem to dilute the punishment of hell – annihilation or distance from God would scarcely seem a disincentive to people who have decided to commit mortal sins after all.  Further, by the Middle Ages the Church realized that dualism supports an utter contempt for the physical body, which can lead people towards extreme and unhealthy asceticism or towards a disregard for the sins of the body and the belief that its sins – sexual sins included – are less significant.

Belief that the body can be re-created in a parallel dimension after death to receive reward or punishment is far preferable.  Seeking a middle-way between the difficulties of basing faith on a future physical resurrection and basing it on dualism and a purely spiritual eternal life, St Thomas Aquinas developed his Theology on the basis of Aristotle’s Philosophy.  Rejecting the dualism proposed by his teacher Plato, in his Metaphysics, Aristotle had set out how all beings have four different types of cause; material causes (physical ingredients), efficient causes (agents), formal cause (what makes something what it is, its definition) and a final cause (the purpose or end to which its existence pertains). Aristotle understood that the soul is the formal cause of the human being, what makes it what it is and defines its existence.  Unlike Plato however, Aristotle did not see the form of a being having any separate metaphysical existence.  The form depends on the materials it specifies, and the end towards which it works.  The soul is, in effect, the function of the body – what Gilbert Ryle later described as “the ghost in the machine” – it gives the impression of being a separate entity but in fact it depends entirely on the physical body for its existence. This is where Aquinas departed from Aristotle; he argued that on death the body is re-created in a parallel heavenly dimension and that the new unity of soul and heavenly body is subject to punishment and reward.  Arguably, this idea of re-creation has a basis in scripture; references such as “They are buried as natural human bodies, but they will be raised as spiritual bodies.” (1 Corinthians 15:42-44) can be interpreted as Biblical support for Aquinas’ “modified dualism”.  Further, John Hick developed a defense of re-creation into a parallel dimension through the “replica theory” which he developed in Death and Eternal Life (1976).  As Hick argued, provided that the replica retains the memories of the original, difficulties with spatio-temporal continuity can be overcome.  When Captain Kirk said “beam me up Scotty!” there was no doubt that Kirk remained Kirk although there was a break in his spatio-temporal existence. Aquinas’ theory of re-creation supports Christian faith far better than either monism and physical resurrection or dualism and purely spiritual reward/punishment.  It avoids both the challenges presented by science and reason to belief in physical resurrection and the theological pitfalls of dualism, while straining credulity to a lesser extent because it requires only that the soul could be briefly separable, not that it must be sustained in a separate state.  For this reason, Aquinas’ theory was adopted into the doctrine of the Roman Catholic Church at the Council of Trent in the 16th Century.

In conclusion, it is clear that orthodox Christian faith does not require belief in a separate soul.  Christian faith can be sustained through a belief in physical resurrection, either in the future starting with the final judgement as the Synoptic Gospels, Creeds and Catechism suggest, or through re-creation into a new body in a parallel dimension as St Thomas Aquinas suggested.  Re-creation does not require belief in a separate soul, but does suppose that the soul is separable.