After all, however, these are matters which have always been regarded by the popes themselves as circumstances of a more or less transitory and accidental character. It is not upon any or all of them that the papacy from first to last has sought to base its high claims to authority. The fundamental explanation, from the papal standpoint, for the peculiar development of the papal power in the person of the bishops of Rome is contained in the so-called theory of the "Petrine Supremacy," which will be found set forth in Pope Leo's sermon reproduced in part below. The essential points in this theory are: (1) that to the apostle Peter, Christ committed the keys of the kingdom of heaven and the supremacy over all other apostles on earth; (2) that Peter, in the course of time, became the first bishop of Rome; and (3) that the superior authority given to Peter was transmitted to all his successors in the Roman bishopric. It was fundamentally on _these_ grounds that the pope, to quote an able Catholic historian, was believed to be "the visible representative of ecclesiastical unity, the supreme teacher and custodian of the faith, the supreme legislator, the guardian and interpreter of the canons, the legitimate superior of all bishops, the final judge of councils--an office which he possessed in his own right, and which he actually exercised by presiding over all ecumenical synods, through his legates, and by confirming the acts of the councils as the Supreme Head of the Universal Catholic Church."[88] Modern Protestants discard certain of the tenets which go to make up the Petrine theory, but it is essential that the student of history bear in mind that the people of the Middle Ages never doubted its complete and literal authenticity, nor questioned that the authority of the papal office rested at bottom upon something far more fundamental than a mere fortunate combination of historical circumstances. Whatever one's personal opinions on the issues involved, the point to be insisted upon is that in studying mediaeval church life and organization the universal acceptance of these beliefs and conclusions be never lost to view. Leo was pope from 440 to 461 and it has been well maintained that he was the first occupant of the office to comprehend the wide possibilities of the papal dignity in the future. In his sermons and letters he vigorously asserted the sovereign authority of his position, and in his influence on the events of his time, as for example the Council of Chalcedon in 451, he sought with no little success to bring men to a general acknowledgment of this authority. Source--Text in Jacques Paul Migne, _Patroligiae Cursus Completus_ ["Complete Collection of Patristic Literature"], First Series, Vol. LIV., cols. 144-148. Translated in Philip Schaff and Henry Wace, _Select Library of Nicene and Post-Nicene Fathers of the Christian Church_ (New York, 1895), Second Series, Vol. XII., pp. 117-118.