"It is done," is a declaration indicating the completion of the work symbolized. It marks the termination of the events of the seventh vial, which are described in the verses following:
"The great city" is "Babylon," (14:8); which "reigneth over the kings of the earth," (17:8); and which John had seen sitting "upon many waters," 17:1. This was doubtless seen when he saw the waters of the symbolic Euphrates being dried up, 16:12. Babylon, being a symbol of the Roman hierarchy, its triple division indicates a like division of the church of Rome, not geographical, but under different leaders, previous to its destruction.
"The cities of the nations," must symbolize other hierarchies, analogous to that of Rome, of which there are the Greek church, in Russia and Greece, the Arminian and Syrian churches, and other corrupt nationalized establishments. All such will become disconnected, like Babylon, with the governments by which they are sustained.
"Great Babylon" then comes into remembrance to drink the cup of the wine of the fierceness of God's wrath. Because her sins have reached unto heaven, "God hath remembered her iniquities," 18:5. This synchronizes with her destruction, symbolized in Rev. 18:8-23. As the Papacy continues till Christ's coming (Dan. 7:21, and 2 Thess. 2:3-8), this epoch must synchronize with that event, when he comes to receive his chosen ones.
With the destruction of Babylon, occurs the subversion of all national authority. As ecclesiastical hierarchies are symbolized by cities, the "mountains" and "islands" on which they are situated must symbolize the larger and smaller governments; and their removal from their places, their subversion in the great moral "earthquake" which is to overwhelm them. This synchronizes with the sixth seal, when they are all "removed out of their places," (6:14); and it leaves the inhabitants of earth in a state of anarchy. It is at this time that the kings and great men of the earth become aware that the great day of God's wrath is come, 6:15-17. With this time of trouble, comes the deliverance of God's people, (Dan. 12:1); who shall be caught up together "to meet the Lord in the air," 1 Thess. 4:17. To them the Lord has said, "Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day; nor for the pestilence that walketh in darkness; nor for the destruction that wasteth at noon-day. A thousand shall fall at thy side, and ten thousand at thy right hand; but it shall not come nigh thee. Only with thine eyes shalt thou behold and see the reward of the wicked. Because thou hast made the Lord which is my refuge, even the Most High, thy habitation," Ps. 91:5-9.