The question of whether Protestant ministers are validly ordained remains a barrier for ecumenical reconciliation between Roman Catholics and Protestants. Because Catholics in the past have judged Protestant ordinations to be invalid, the Catholic Church in the present feels bound to name these communions "not fully-churches." Many Protestants, however, accept Catholic bishops, priests, and deacons as ministers of the gospel and the Catholic Church as a true church (albeit one in need of ongoing reformation).

Since the problem is primarily a Catholic one, any reconciliation will require that Catholics find a solution through the theological resources of their own tradition. In An Ecumenical Priesthood, Karl Rahner proposes that the nature of the church and the affirmation of the presence of grace among Protestants may open a door to renewal and healing. As canon law validates improperly contracted marriages by examining their fruits, so recognizing the spiritual fruits of Protestant sacraments could allow Catholics to "restipulate" their position on these sacramental acts (and thereby the validity of the ministers who perform them), without revising the Church's original judgment.

Because the book is now nearly fifty years old and deals with internal Catholic questions, it is offered with an introduction to the era and an analysis of the argument, as well as an overview of recent decades of ecumenical discussions.