This is a sequel to Walls's Heaven: the Logic of Eternal Joy (2002) and his earlier book Hell: The Logic of Eternal Damnation (1992). With Purgatory, Walls completes his examination of the Christian theology of the afterlife. He sketches the theological rationale for the doctrine of purgatory and traces its development in Roman Catholic theology. He examines Protestant objections to the doctrine - in particular the claims that it is not scriptural - and the Protestant Reformed and Wesleyan alternatives.

As an intermediate state between death and resurrection, purgatory has traditionally assumed body-soul dualism, a concept called into question by much recent theology. Walls considers the impact of a monistic view of the human person on the doctrine of purgatory. In Catholic theology, purgatory has been understood as applying to persons who are already in a state of grace or salvation. In popular thought, however, it is sometimes understood as a "second chance" for the unconverted. Walls examines the doctrine in the light of "inclusive" theories of salvation that allow for repentance and conversion after death. He concludes with an examination of C.S. Lewis's writings on purgatory, and suggests that Lewis can be a model for evangelicals and other Protestants to engage the doctrine of purgatory in a way that is true to their theology.