Revelation 17:8

The beast which thou sawest was, and is not
It is added at the end of the verse where the same description is given, "and yet is"; this beast is to be understood not of the devil, who "was" the god of this world, "is not", being cast out by Christ, and yet is in being; for he, the dragon, is distinguished from this beast, and indeed from him the beast has his seat, power, and authority, ( Revelation 13:1 Revelation 13:2 ) nor any particular emperor, as Domitian, a cruel and savage one, who was in power in Vespasian's time, when he was abroad, and then was out of it upon his return, and yet afterwards was in again, being as one sent from hell, and went at last into perdition; but the Roman empire itself is intended, as we have seen, which carried and supported the Papacy; and variously may this be interpreted; as that it was in the hands of the Romans originally, and long continued with them, but now "is not", in John's time, being in the hands of Trajan, a Spaniard, "and yet is" in being, Rome being the metropolis of it: it was a very powerful and flourishing empire, "and is not", being destroyed by the Goths and Vandals, "and yet is" a large empire under the jurisdiction of antichrist; it was an idolatrous empire, which encouraged the worshipping of Heathen gods, "and is not" guilty of the same idolatrous practices it was, as the worshipping of Jupiter, Mars, &c. "and yet is" much given to idolatry in another way, worshipping the virgin Mary, and other saints; it "was" under kings, consuls, dictators, tribunes, decemvirs, and emperors, it "is not" in such a form of government, "and yet is" under its seventh head the pope; "Rome was", but "is not" the same it has been, in some respects, and yet is the same, for idolatry, cruelty it has the image of old Rome, when Pagan, and under the power of the dragon; and though antichrist was not risen up in the empire to an head, to supreme power yet it was in being in the apostles' times, and began to work, and by degrees to show itself. In short, the meaning is, that this beast "was" the Roman empire, as Pagan, described by a dragon with seven heads, and ten horns, with crowns on the heads, but is not as yet in the Apostle John's time, as Papal, described with seven heads, and ten horns, and crowns on the horns.

And shall ascend out of the bottomless pit:
out of hell, from whence the antichristian beast has its power; see ( Revelation 11:8 ) ( 13:2 ) otherwise all civil power is from God, but not antichristian power, that is from the devil:

and go into perdition;
everlasting destruction, the lake which burns with fire and brimstone; this will be the end of the beast, ( Revelation 19:20 ) .

and they that dwell on the earth shall wonder;
the inhabitants of the Roman empire, earthly minded men, shall have the beast in great veneration, and follow and worship him; see ( Revelation 13:3 Revelation 13:4 Revelation 13:12 )

whose names were not written in the book of life from the foundation
of the world;
none but reprobates, not any of the elect of God, are the admirers and worshippers of antichrist; (See Gill on Revelation 13:8)

when they beheld the beast that was, and is not, and yet is;
the Roman empire in glory, under the emperors, destroyed by the Goths, and revived in the Papacy. The Vulgate Latin and Ethiopic versions leave out the last clause,

and yet is.