The identical phrase described Johns journey with the angel who showed him the Harlot (Rev. Rev. 17:3+). An intentional contrast is being made between the Harlot, the great city Babylon, and the bride, the great city, the New Jerusalem. It is the Holy Spirit who transports John while the angel journeys with him. See commentary on Revelation 17:3. John was in the Spirit on the Lords Day when he had his first vision of the glorified Lord (Rev. Rev. 1:10+). He was also in the Spirit when he was called up to the throne room to see the things yet future (Rev. Rev. 4:2+). See commentary on Revelation 4:2.
to a great and high mountain
In the twenty-fifth year of the Babylonian captivity, the hand of the LORD was upon Ezekiel and took him in the visions of God to a very high mountain upon which there was something like the structure of a city (Eze. Eze. 40:2). Ezekiel was shown the millennial Jerusalem, not the New Jerusalem as here. Like John, he was attended by an angel (man) with a measuring rod who then measured aspects of the city for Ezekiel to record.1 The high mountain may merely be the vantage point from which John is shown the holy Jerusalem. Or, it may reflect the terrain upon which the holy city alights. This would correspond to the mountain of the Lords house, the site of the Temple during the Millennial Kingdom (Isa. Isa. 2:2-4; Zec. Zec. 8:3-4). This seems likely because, as in the Millennium, a river goes forth from beneath the throne (Rev. Rev. 22:1+). Thus, the area of the New Jerusalem must be elevated above the surrounding region.
the great city, the holy Jerusalem, descending out of heaven from God
Descending is καταβαίνουσαν [katabainousan] , present tense: while coming down. John saw the city as it was in the process of descending. It appears John sees the city descend twice. First, he sees it descending as part of the general presentation of the eternal order (Rev. Rev. 21:2+). Now, the vision recapitulates the descent prior to a detailed tour with the angel. The appearance of the citys descent twice in the same chapter does not necessitate taking the descents as two different points in history (one a millennial descent, the other in the eternal state). As we discussed above, lack of the curse (Rev. Rev. 22:2+) is conclusive evidence against any idea that Revelation Rev. 21:9+-Rev. 22:7+ concerns the New Jerusalem in the Millennium. See commentary on Revelation 21:9.