This work is an attempt to tell the story of the first part of the life and times of one of the most picturesque and remarkable men of the Middle Ages, Baldassare Cossa, who on the 17th May 1410 became Pope John the Twenty-third. In order accurately to appreciate the career and character of this churchman, it is necessary to have in the first place a clear perception of the state of the Church at the end of the fourteenth century, and in the second to realise its position with regard to the Empire. For the continued existence of the Holy Roman Empire and its connection with the Holy Roman Church are the two cardinal points of medieval history in Europe. In the days of Constantine the Great, and for some centuries after, the Empire and the Church were practically conterminous; they represented the same body of people under different aspects; they were the civil and religious sides of Christendom. Later on they were typified as the Sun and the Moon in the heavens, as the Two Swords delivered to Saint Peter; but the theory of their interdependence was not fully worked out until their connection was no longer unchallenged as formerly. The connection of the Empire and the Church gave rise to those two wonderful fictions known as the Donation of Constantine and the Translation of the Empire. When Constantine the Great moved the seat of Empire to Byzantium (327), it was said that he gave Italy, some even maintained that he made over all the nations of Western Europe, to Pope Sylvester and his successors; this was the so-called Donation of Constantine.