6 Listen now to a deep mystery. Rahab is there through Him, through whom also is Babylon, now no longer Babylon, but beginning to be Jerusalem. The daughter is divided against her mother, and will be among the members of that queen to whom is said, “Forget your own people, and your father's house, so shall the king have pleasure in your beauty.” For how could Babylon aspire to Jerusalem? How could Rahab reach those foundations? How could the Philistines, or Tyre, or the people of the Ethiopians?
Listen to this verse, “Sion, my mother, a man shall say.” There is then a man who says this: through whom all those I have mentioned make their approach. Who is this man? It tells if we hear, if we understand. It follows, as if a question had been raised, through whose aid Rahab, Babylon, the Philistines, Tyre, and the Morians, gained an entrance. Behold, through whom they come; “Sion, my mother, a man shall say; and a man was born in her, and Himself the Most High has founded her”.
What, my brethren, can be clearer? Truly, because “very excellent things are spoken of you, thou city of God.” Lo, “Sion, O mother, a man shall say.” What man? “He who was born in her.” It is then the man who was born in her, and He Himself has founded her. Yet how can He be born in the city which He Himself founded? It had already been founded, that therein He might be born. Understand it thus, if you can. “Mother Sion, he shall say;” but it is “a man” that “shall say, Mother Sion; yea, a man was born in her:” and yet “he has founded her” (not a man, but), “the Most High.”
As He created a mother of whom He would be born, so He founded a city in which He would be born. What hope is ours, brethren! On our behalf the Most High, who founded the city, addresses that city as a mother: and “He was born in her, and the Most High has founded her.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)