7 Evidently this description cannot be taken literally (in fact, it is absurd to suppose a city the length, breadth and height of which are all twelve thousand furlongs), and therefore the details of it must be mystically understood. The great city which Cain first built and called after his son must be taken to represent this world, which the devil, that accuser of his brethren, that fratricide who is doomed to perish, has built of vice cemented with crime, and filled with iniquity.
Therefore it is spiritually called Sodom and Egypt. Thus it is written, “Sodom shall return to her former estate,” that is to say, the world must be restored as it has been before. For we cannot believe that Sodom and Gomorrha, Admah and Zeboim are to be built again: they must be left to lie in ashes forever. We never read of Egypt as put for Jerusalem: it always stands for this world. To collect from Scripture the countless proofs of this would be tedious: I shall adduce but one passage, a passage in which this world is most clearly called Egypt.
The apostle Jude, the brother of James, writes thus in his catholic epistle: “I will, therefore, put you in remembrance, though ye once knew this how that Jesus, having saved the people out of the land of Egypt, afterward destroyed them that believed not.” And, lest you should fancy Joshua the son of Nun to be meant, the passage goes on thus: “And the angels which kept not their first estate, but left their own habitation, he has reserved in everlasting chains, under darkness, unto the judgment of the great day.” Moreover, to convince you that in every place where Egypt, Sodom and Gomorrha are named together it is not these spots, but the present world, which is meant, he mentions them immediately in this sense.
“Even as Sodom and Gomorrha,” he writes, “and the cities about them, in like manner giving themselves over to fornication and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire.” But what need is there to collect more proofs when, after the passion and the resurrection of the Lord, the evangelist Matthew tells us: “The rocks rent, and the graves were opened; and many bodies of the saints which slept arose and came out of the graves after his resurrection, and went into the holy city and appeared unto many”? We must not interpret this passage straight off, as many people absurdly do, of the heavenly Jerusalem: the apparition there of the bodies of the saints could be no sign to men of the Lord's rising.
Since, therefore, the evangelists and all the Scriptures speak of Jerusalem as the holy city, and since the psalmist commands us to worship the Lord “at his footstool;” allow no one to call it Sodom and Egypt, for by it the Lord forbids men to swear because “it is the city of the great king.”
Source: Letters (New Advent)