Pope Leo XIII cites Saint Augustine's prescient description of the 'two cities' in the Encyclical Humanum genus:
You can read the rest of Humanum genus at Papal Encyclicals Online.
1. The race of man, after its miserable fall from God, the Creator and the Giver of heavenly gifts, "through the envy of the devil," separated into two diverse and opposite parts, of which the one steadfastly contends for truth and virtue, the other of those things which are contrary to virtue and to truth. The one is the kingdom of God on earth, namely, the true Church of Jesus Christ; and those who desire from their heart to be united with it, so as to gain salvation, must of necessity serve God and His only-begotten Son with their whole mind and with an entire will. The other is the kingdom of Satan, in whose possession and control are all whosoever follow the fatal example of their leader and of our first parents, those who refuse to obey the divine and eternal law, and who have many aims of their own in contempt of God, and many aims also against God.
2. This twofold kingdom St. Augustine keenly discerned and described after the manner of two cities, contrary in their laws because striving for contrary objects; and with a subtle brevity he expressed the efficient cause of each in these words: "Two loves formed two cities: the love of self, reaching even to contempt of God, an earthly city; and the love of God, reaching to contempt of self, a heavenly one."[De civ. Dei, 14, 28 (PL 41, 436)] At every period of time each has been in conflict with the other, with a variety and multiplicity of weapons and of warfare, although not always with equal ardor and assault. At this period, however, the partisans of evil seems to be combining together, and to be struggling with united vehemence, led on or assisted by that strongly organized and widespread association called the Freemasons. No longer making any secret of their purposes, they are now boldly rising up against God Himself. They are planning the destruction of holy Church publicly and openly, and this with the set purpose of utterly despoiling the nations of Christendom, if it were possible, of the blessings obtained for us through Jesus Christ our Savior. Lamenting these evils, We are constrained by the charity which urges Our heart to cry out often to God: "For lo, Thy enemies have made a noise; and they that hate Thee have lifted up the head. They have taken a malicious counsel against Thy people, and they have consulted against Thy saints. They have said, 'come, and let us destroy them, so that they be not a nation'."[Ps. 82:24]
3. At so urgent a crisis, when so fierce and so pressing an onslaught is made upon the Christian name, it is Our office to point out the danger, to mark who are the adversaries, and to the best of Our power to make head against their plans and devices, that those may not perish whose salvation is committed to Us, and that the kingdom of Jesus Christ entrusted to Our charge may not stand and remain whole, but may be enlarged by an ever-increasing growth throughout the world.