The "City of God" is the masterpiece of the greatest genius among the Latin Fathers, and the best known and most read of his works, except the "Confessions." It embodies the results of thirteen years of intellectual labor and study (from A.D. 413-426). It is a vindication of Christianity against the attacks of the heathen in view of the sacking of the city of Rome by the barbarians, at a time when the old Græco-Roman civilization was approaching its downfall, and a new Christian civilization was beginning to rise on its ruins. It is the first attempt at a philosophy of history, under the aspect of two rival cities or communities- the eternal city of God and the perishing city of the world. This was the only philosophy of history known throughout Europe during the middle ages; it was adopted and reproduced in its essential features by Bossuet, Ozanam, Frederick Schlegel, and other Catholic writers, and has recently been officially endorsed, as it were, by the scholarly Pope Leo XIII. in his encyclical letter on the Christian Constitution of States (Immortale Dei, Nov. 1, 1885); for the Pope says that Augustin in his De Civitate Dei, "set forth so clearly the efficacy of Christian wisdom and the way in which it is bound up with the well-being of States, that he seems not only to have pleaded the cause of the Christians of his own time, but to have triumphantly refuted the false charges [against Christianity] forever."