Albert Barnes on Revelation 18:21

And a mighty angel. Barnes on "Re 18:1" This seems, however, to have been a different angel from the one mentioned in Rev 18:1, though, like that, he is described as having great power.

Took up a stone like a great millstone. On the structure of mills among the ancients, Mt 24:41.

And cast it into the sea. As an emblem of the utter ruin of the city; an indication that the city would be as completely destroyed as that stone was covered by the waters.

Saying, Thus with violence. With force--as the stone was thrown into the sea. The idea is, that it would not be by a gentle and natural decline, but by the application of foreign power. This accords with all the representations in this book, that violence will be employed to overthrow the Papal power. See Rev 17:16-17. The origin of this image is probably Jer 51:63-64: "And it shall be, when thou hast made an end of reading this book, that thou shalt bind a stone to it, and east it into the midst of Euphrates; and thou shalt say, Thus shall Babylon sink, and shall not rise from the evil that I will bring on her."

(a) "Thus with violence" Jer 51:64