And I saw an angel come down from heaven - Compare the notes on Revelation 10:1. He does not say whether this angel had appeared to him before, but the impression is rather that it was a different one. The whole character of the composition of the book leads us to suppose that different angels were employed to make these communications to John, and that, in fact, in the progress of things disclosed in the book, he had contact with a considerable number of the heavenly inhabitants. The scene that is recorded here occurred after the destruction of the beast and the false prophet Revelation 19:18-21, and therefore, according to the principles expressed in the explanation of the previous chapters, what is intended to be described here will take place after the final destruction of the papal and Muhammedan powers.
Having the key of the bottomless pit - See the notes on Revelation 1:18; Revelation 9:1. The fact that he has the key of that underworld is designed to denote here, that he can fasten it on Satan so that it shall become his prison.
And a great chain in his hand - With which to bind the dragon, Revelation 20:2. It is called great because of the strength of him that was to be bound. The chain only appears to have been in his hand. Perhaps the key was suspended to his side.