Karl Barth, because of poor health, was unable to accept the invitation to be an observer at the final two sessions of Vatican II. When his condition improved, however, he was eager to make a trip to Rome to learn firsthand how the Council decisions were understood at the heart of Catholicism. The Secretariat for Christian Unity readily agreed, and so Barth made his pilgrimage "to the threshold of the Apostles" September 22-29, 1966. Included here are an account of Barth's trip to Rome, questions for clarification and critical questions he asked, an essay about the Constitution on Divine Revelation, a letter on Mariology to a German Catholic theologian, and an appendix on Barth's thoughts about Vatican II before its work was completed. He calls for Protestants to take a lesson from the stirrings of renewal within the Roman Church and "sweep away the dust before the door of our own church with a careful but nevertheless mighty broom."