Revelation 18:21

And a strong angel took up a great stone as it were a great millstone and cast it into the sea, saying, Thus with a mighty fall shall Babylon, the great city, be cast down, and shall be found no more at all.

A strong angel ... stone ... cast into the sea ... This is the final judgment of Babylon, the great city, the latter words showing that more than just pagan Rome is meant; this is the final day, the ultimate judgment of the great day of God at the Second Advent of Christ. The Biblical background of the figure is in Jeremiah 51:59-64:

Jeremiah instructed Seriah who was traveling to Babylon to take a scroll of Jeremiah's prophecy of the destruction of Babylon, and upon his arrival in Babylon to read it to the city, and then to throw this scroll, weighted with a stone, into the Euphrates river. As it sinks, he is to prophesy that thus shall Babylon sink to rise no more (paraphrase).

The significant thing is that the prophecy was literally and summarily fulfilled; the old Babylon sank into oblivion, and even the ancient site of it is today not certainly known. A similar finality of the overthrow of mystical Babylon is indicated by the employment of the same imagery here. The choice of the figure demands this conclusion; therefore, this is not merely a temporary judgment. The final end at the last judgment is indicated. The placement of the verbs in the Greek indicates "the final act of judgment."[64] This verse is a reiteration of Revelation 16:19,20.

And shall be found no more at all ... The ominous phrase "no more at all" occurs six times in this chapter, five times in these last verses, indicating the absolute finality of the judgment. It is impossible to limit the application of this merely to pagan Rome. "This judgment is sudden, complete and final."[65] Let those who deny that such a thing as this will take place show us the site of the old Babylon. Where is it? Earth knows not even the place. Exactly the same word of God that doomed the old one, doomed also the one in view here.

[64] R. C. H. Lenski, op. cit., p. 530.

[65] Frank L. Cox, op. cit., p. 109.