Having explained the building up of the city of God, it only remains to set forth briefly what is chiefly required for citizens to be enrolled in that most happy City. And this can be said in one word: namely, that we renounce the city of this world, and that we live here in the meantime as strangers and pilgrims.
For it cannot come to pass that we should be at once citizens of the world and citizens of the blessed; and there is no one who tenders his renunciation to the city of this world without being received at once into the bosom of the city of God. But let us set forth the whole matter a little more fully. Two cities are declared to us in the holy Scriptures: an earthly city, which began with Cain, who first founded a city upon the earth, as we read in Genesis; and a heavenly city, which began with Abel, and whose founder is not Abel but God, as we said above on the word of the Apostle (Gen. 4). Of the former, great Babylon bore the figure — Babylon, which signifies Confusion; of the latter, Jerusalem, the city of the great King, which is called the Vision of peace. The citizens of the earthly city are those who dwell upon the earth not only in body but also in heart: they till the earth, they gape after earthly goods, for them they contend, for them they go to law, for them they stir up tumults. The prince of this city is the Devil, who, cast out of the heavenly city, seized the tyranny of the earthly city. For although our Lord, when His passion was at hand, said: "Now is the judgment of the world: now shall the prince of this world be cast out" (John 12), and although He did in truth cast him out with the rod of His cross, and through that same cross triumphed over him — as the Apostle says in the Epistle to the Colossians: "And despoiling the principalities and powers, He hath exposed them confidently in open show, triumphing over them in Himself" (Col. 2) — yet this is not to be understood as though the devil were utterly cast out of the world, or had altogether lost the principality of the world, but rather that he was cast out from all those, and in all those lost his principality, who join themselves to Christ and, fleeing from the earthly city, are enrolled in the heavenly city. That the Devil still holds dominion in the earthly city, which is also called the world, the Apostle teaches in the Epistle to the Ephesians when he says: "Our wrestling is not against flesh and blood, but against principalities and powers, against the rulers of the world of this darkness" (Eph. 6). Therefore Satan still holds principality in the world together with his ministers, and he is the ruler of the world — that is, of worldly men and of the citizens of the earthly city — of which world St. John says: "The whole world is seated in wickedness"; as if he had said: the world cleaves to its head, who is called by antonomasia the wicked one; or, the world is under the rule and power of the wicked demon.
Furthermore, the citizens of the heavenly city are the blessed who reign in the kingdom of God, and also those who, placed in a mortal body, inhabit the earth — not in heart but in body; for in heart their conversation is in heaven (Phil. 3), and they desire to be dissolved and to be with Christ, who is the king of the heavenly city (Phil. 1). But because the citizens of heaven are mingled on earth with the citizens of earth, the holy Scriptures say that the citizens of heaven are in the world but not of the world, and that they are in the world not as citizens but as strangers and pilgrims; for so speaks St. Peter: "I beseech you, as strangers and pilgrims, to refrain yourselves from carnal desires" (1 Pet. 2). And of the citizens of earth, on the contrary, the sacred letters say that they are "strangers to the testaments, having no hope of the promise, and without God in this world"; for these are the words St. Paul uses in the Epistle to the Ephesians (Eph. 2). Since these things are so, let no one deceive himself; let no one think that he can be at the same time a citizen of the world and a citizen of heaven. The citizens of the world are of the world; the citizens of heaven are not of the world. To be of the world and not to be of the world are contradictories; therefore they cannot be joined together. Wherefore let those whom the world and earthly things still delight not imagine that they can have a place in the heavenly city, unless they first go out of the world, renounce the present age, and disdain earthly things.
And because these are great things, and are understood by few, or pondered as they ought to be, therefore — that no one may plead ignorance on the last day — there is nothing that the Apostles and Evangelists inculcate and repeat more often. Hear the Lord: "You are of this world, I am not of this world" (John 8). Again, to the Apostles: "If you had been of the world, the world would love its own; but because you are not of the world, therefore the world hateth you" (John 15). Hear St. Paul: "The wisdom of this world is foolishness with God" (1 Cor. 3). Again: "You should have gone out of this world" (1 Cor. 5). Again: "That we be not condemned with this world" (1 Cor. 11). Hear St. James: "Know you not that the friendship of this world is the enemy of God? Whosoever therefore will be a friend of this world becometh an enemy of God" (Jas. 4). Hear St. Peter: "Flee the corruption of that concupiscence which is in the world" (2 Pet. 1). Hear St. John: "Love not the world, nor the things which are in the world" (1 John 2). Again: "If any man love the world, the charity of the Father is not in him." Again: "And the whole world is seated in wickedness" (1 John 5). Finally, hear the Lord saying in His prayer to the Father: "I pray for them; I pray not for the world, but for them whom Thou hast given Me. And the world hath hated them, because they are not of the world, as I also am not of the world" (John 17). Here we have it most plainly: the world is, in a manner, so excommunicated and condemned by God that Christ judged He should in no wise pray for it. But if Christ does not pray for the world, how is it that elsewhere He says: "God so loved the world, as to give His only begotten Son" (John 3)? Does the Father love the world, and the Son hate the world? Or how does the Son exclude from His prayer the world which the Father does not exclude from His love? St. Augustine (Tract. CVII on John), expounding this latter passage, says that the world for which Christ says He does not pray signifies only the reprobate, as the Apostle speaks in the first Epistle to the Corinthians when he says: "that we be not condemned with this world" (1 Cor. 11). But we can also say that Christ did not pray for the world because the things He was then asking for the Apostles were in no way suited to the world. For He was asking the gift of perseverance: "Keep them," He says, "in Thy name" (John 17); and at the same time He was asking eternal glory for them, when He said: "Father, I will that where I am, they also may be with Me, that they may see My brightness." But these things do not suit the world; for the world is not fitted for the kingdom of God unless it be first made clean — just as one would not say that a man spattered all over with mud, and filthy, should enter the king's bedchamber. God, however, loves the world, and gave His Son for it, that He might truly cleanse it and make it fit for the kingdom. For this reason also Christ prayed for His crucifiers — not that they should persevere in the state in which they were, but that the Father would forgive them, and by forgiving them cleanse them, so that they too might go out of the world. And this Christ also observed in that prayer in which He said: "I pray not for the world"; for He adds, not far below: "That the world may believe that Thou hast sent Me." The sum, therefore, is that Christ prayed for His own, not for the world; because unless a man go out of the world before he goes out of the body, he will never come to the kingdom of God.
Therefore, whoever loves that heavenly city, let him make haste to go out of the world, lest the last day come suddenly and snatch him away from life, when there will be no hope of conversion for him; and when he has gone out of the world, let him forget the world and its concupiscences, that he may continually remember the city of the Lord alone, and swear with the holy Prophet: "If I forget thee, O Jerusalem, let my right hand be forgotten; let my tongue cleave to my jaws, if I do not remember thee, if I make not Jerusalem the beginning of my joy" (Ps. 136). For this is the true mark of the citizens of the eternal City: if truly and with all their heart they would choose rather to be deprived of tongue and hands than to say or do anything against the love of God the Father and of the heavenly fatherland; and if truly, not feignedly, that City which makes her citizens blessed is the beginning of their joy — so that no worldly happiness delights them, and the remembrance and expectation of future joys alone suffices to gladden them in this exile.
It is fitting to conclude this book with the words of St. Augustine, so that those who perhaps will believe nothing may not hesitate to put faith in so great a man. For in his exposition of the sixty-first Psalm he explains in these words what is the true mark of the citizens of the city of the world and of the citizens of the City of God: "All," he says, "who savor earthly things, all who prefer earthly happiness to God, all who seek their own and not the things of Jesus Christ, belong to that one city which is called mystically Babylon, and which has the Devil for its king; but all who savor the things that are above, who meditate on heavenly things, who live in this world with anxious care lest they offend God, who guard against sinning, and whom it does not shame to confess when they have sinned — the humble, the meek, the holy, the just, the pious, the good — all belong to one City, which has Christ for its king."