Revelation CHAP. II. What is commaunded to bee written to the Angels, that is, the Ministers of the Churches of 1 Ephesus, 8 Smyrna, 12 Pergamus, 18 Thyatira: and what is commended, or found wanting in them.
Revelation 2:12-13 12 And to the Angel of the Church in Pergamos, write, These things saith hee, which hath the sharpe sword with two edges: 13 I know thy workes, and where thou dwellest, euen where Satans seat is, and thou holdest fast my Name, and hast not denied my faith, euen in those daies, wherein Antipas was my faithful Martyr, who was slaine among you, where Satan dwelleth.
Revelation CHAP. XIII. 1 A beast riseth out of the sea with seuen heads and ten hornes, to whom the dragon giueth his power. 11 Another beast commeth vp out of the earth: 14 causeth an image to be made of the former beast, 15 and that men should worship it, 16 and receiue his marke.
Revelation 13:2 KJV  And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon gave him his power, and his seat, and great authority.
Assuming then that the dragon mentioned in Rev 13:2 is Satan mentioned in Rev 2:13 we conclude that the beast of Rev 13:2 will rule from Pergamos mentioned in Rev 2:12, which also is called Pergamus in the chapter summary of Revelation 2.
So where is Pergamos/Pergamus?
Some people would claim that it is Pergamum in Turkey. This is the established opinion among Christians.
But some others would say that it is Pergamus on Cyprus.
But it could also be Pergamus on Crete.
Some people believe that this letter to the church of Pergamos is not a future prophecy but rather something that happened in the first century. Other people believe that this is part of a future prophecy.