The mystery of the Church-Christ's continuing presence on earth-is that it is at once divine and human. Its mission, defined by its founder, imposes on it strict duties of unity, teaching,and sanctity. The limits of my study are therefore clearly defined: it will pursue the fulfillment of each of these requirement in the Church's historic evolution. Repetitions, have inevitably resulted, for the same event often fulfills each requirement. Far from rejecting the repetitions, it seemed to me that it was necessary to maintain them and sometimes to emphasize them; they constitute a way of making tangible the decisive elements in the history of the church