Aware of the greatness of the task which confronted him in the fifth century with the great upheaval caused everywhere in Europe by the invasion of the Barbarians, St. Leo guided the destiny of the Chruch with such trust in God and wideness of view that he became not only the guardian of the faith but the savior of western civilization. His encounter with Attila, “the scourge of God,” has remained famous in history. His interventions in the doctrinal controversies of the time were no less important. It was at his instance that the Council of Chalcedon, in 451, defined the existence of the two natures, divine and human, in the one Person of Christ. He died in 461. Many of the Collects in the Missal were composed by St. Leo and are some of the most beautiful that we have.