St. Augustine wrote the City of God at a time of trouble for the Roman Empire. Pope John XXIII issued his Pacem in Terris “ to all men of good will“ in an age of universal conflict. While Augustine's philosophy of history includes a conception of an ideal Christian commonwealth, the predominant theme in his great book is pessimism regarding the City of Man. The strongest motif in the late Pope's encyclical, on the other hand, is optimism concerning the power of responsible men to shape the future in a favorable way.