Rev 17:3-5
"And he carried me away in the Spirit into a wilderness, and I saw a woman sitting on a scarlet beast that was full of blasphemous names, and it had seven heads and ten horns... And on her forehead was written a name: Mystery, Babylon the Great, the Mother of Prostitutes and of the Abominations of the Earth."
Babylon is the symbol of imperial Rome for the first readers, but it transcends any single historical identification. It is the human power system that opposes God: seductive (prostitute), drunk with the blood of the saints, sustained by political power (the beast). Every system that demands absolute loyalty is Babylon.