In this book, Leithart argues that ancient, modern, and postmodern literature is all characterized by a tragic view of history. For Leithart, such a view of history has characters that generally begin well but inescapably descend to a bad end. History is seen as tragic when it moves from good to bad with no hope of rescue.
In contrast, Leithart argues that Christianity uniquely advances a comic view of history. Such a view acknowledges human depravity yet holds out hope for redemption and restoration, not only of the human race but of all things. The Christian view of history claims that the world is the good creation of a good God who has good plans for what he has made.
The difference is the Christian doctrine of the God who is triune. The point can be illustrated by considering that the god of Plato was ultimately and perfectly One. Thus, any creative departure necessarily meant imperfection. That which is supplemental to the One is necessarily incomplete. The Christian doctrine of God, though, has a God who is a unity of persons in relation. The Son has his origin in the Father but he is no less perfect and no less divine than the Father because they share the same essence. The Spirit issues from the Father and the Son and, once again, is no less perfect than his origin. Christianity, thus, has a doctrine of God where the supplement is in no way ontologically inferior to the origin. This yields a view of creation and history in which the triune God creates, but the creation is not necessarily a departure from the goodness of the perfect creator. Christianity has a metaphysic in which there is hope of perfection for that which is supplemental to the origin, thus the Christian comic view of history in which things end well despite human rebellion and sin. Ultimately, Leithart argues that any comic view of history and positive eschatology must be grounded in the Christian doctrine of the Trinity.
Much more could be said about this book, and the issues it raises create manifold opportunities for further study. Let me conclude by saying that this is a deeply satisfying book that comes with my highest recommendation.