The word “as” indicates that the reference to the bride is an analogy. The force of the analogy is caught in the word “prepared.” As her attendants ensure that a bride prior to her coming down the aisle is dressed in a manner befitting her meeting the bridegroom, so also God had taken time prior to the city’s descent from heaven to ensure that she was properly outfitted for the occasion. He, after all, will live in this city! The preparation we might think of revolves around the work of our Lord Jesus Christ when he laid down his life on the cross to reconcile (earthly) sinners to (heavenly) God. The analogy of the bride further recalls the vision John shared in Revelation 19:9 about the marriage supper of the Lamb.
2 And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.