The most fundamental theme of Nygren’s work treats one of the central questions of the Christian tradition: How much of this tradition is uniquely Christian and how much of it is a product of the classical culture in which Christianity developed and out of which the intellectual heritage of Western civilization emerged? A related question is whether the values of Christianity are essentially the same as those held by pagans such as Plato and Aristotle or whether Christianity introduced a radically new set of values. Nygren answers that from the very earliest years Christianity has absorbed non-Christian ideas. Moreover, as the values associated with the Christian and non-Christian are utterly different, the values and views of Greco-Roman antiquity have introduced alien elements into Christianity.
In addition, Nygren identifies the concept of love as a fundamental motif, the distinguishing idea of Christianity. He recognizes that it is not always clear just what love means, though, and that dissimilar kinds of forces are identified by the use of the single English word. His contribution is to carefully consider the nature of Christian love and to distinguish it from other views of love.
The attempt to identify the uniquely Christian idea of love and to trace the intellectual history of this idea involves Nygren in a central confessional dispute. One of the Protestant objections to Catholicism was that the Catholic intellectual tradition had absorbed non-Christian influences and had therefore moved away from true Christianity. The Catholic hierarchy, similarly, was seen as a human effort to create a link to God through the Church structure, in place of the immediate descent of God’s love and grace to each individual. While Nygren does not explicitly criticize Catholicism, the ultimate characterization of Luther’s teachings as the return to agape makes Nygren a sophisticated advocate for the Protestant side of the Catholic-Protestant debate.