Babylon the great
What the harlot city names — Rome, Jerusalem, or a power still to come.
Revelation 17:1-18
1One of the seven angels who had the seven bowls came and spoke with me, saying, “Come here. I will show you the judgment of the great prostitute who sits on many waters,
2with whom the kings of the earth committed sexual immorality. Those who dwell in the earth were made drunken with the wine of her sexual immorality.”
3He carried me away in the Spirit into a wilderness. I saw a woman sitting on a scarlet-colored beast, full of blasphemous names, having seven heads and ten horns.
4The woman was dressed in purple and scarlet, and decked with gold and precious stones and pearls, having in her hand a golden cup full of abominations and the impurities of the sexual immorality of the earth.
5And on her forehead a name was written, “MYSTERY, BABYLON THE GREAT, THE MOTHER OF THE PROSTITUTES AND OF THE ABOMINATIONS OF THE EARTH.”
6I saw the woman drunken with the blood of the saints, and with the blood of the martyrs of Jesus. When I saw her, I wondered with great amazement.
7The angel said to me, “Why do you wonder? I will tell you the mystery of the woman, and of the beast that carries her, which has the seven heads and the ten horns.
8The beast that you saw was, and is not; and is about to come up out of the abyss and to go into destruction. Those who dwell on the earth and whose names have not been written in the book of life from the foundation of the world will marvel when they see that the beast was, and is not, and shall be present.
9Here is the mind that has wisdom. The seven heads are seven mountains on which the woman sits.
10They are seven kings. Five have fallen, the one is, the other has not yet come. When he comes, he must continue a little while.
11The beast that was, and is not, is himself also an eighth, and is of the seven; and he goes to destruction.
12The ten horns that you saw are ten kings who have received no kingdom as yet, but they receive authority as kings with the beast for one hour.
13These have one mind, and they give their power and authority to the beast.
14These will war against the Lamb, and the Lamb will overcome them, for he is Lord of lords, and King of kings, and those who are with him are called chosen and faithful.”
15He said to me, “The waters which you saw, where the prostitute sits, are peoples, multitudes, nations, and languages.
16The ten horns which you saw, and the beast, these will hate the prostitute, will make her desolate, will strip her naked, will eat her flesh, and will burn her utterly with fire.
17For God has put in their hearts to do what he has in mind, to be of one mind, and to give their kingdom to the beast, until the words of God should be accomplished.
18The woman whom you saw is the great city, which reigns over the kings of the earth.”
What each tradition says
Rabbinic / Midrashic
The Jewish Encyclopedia identifies the harlot Babylon as Rome. It says the kings of the earth repeat Ezekiel’s lamentation over the fall of Tyre when they mourn Babylon’s fall.
Jewish Encyclopedia (1905), article "Revelation (Book of)", The Jewish Encyclopedia (Funk & Wagnalls, 1901-1906) — "Rome the Great Harlot" (on xvii.-xix.)
Patristic
Victorinus of Pettau states that the seven heads are the seven hills on which the woman sits. He takes this to mean the city of Rome.
Victorinus of Pettau, Commentary on the Apocalypse of the Blessed John — ANF vol. 7 (ed. Roberts & Donaldson) — From the seventeenth chapter (on Rev. xvii. 9)
Catholic
Challoner's note says Babylon is either the city of the devil in general or, if the place is understood as a particular city, pagan Rome. In that second reading, pagan Rome persecuted the Church for 300 years and was the principal seat of empire and idolatry.
George Leo Haydock (citing Challoner), George Haydock's Catholic Bible Commentary (1859) — On Revelation 17:5 (Challoner's note)
Orthodox
No sourced position found. Checked CCEL NPNF 2nd series (Cyril of Jerusalem npnf207, Gregory of Nyssa npnf205) and the Schaff ANF corpus for a non-Latin author on Rev 17 — only the Latin Victorinus comments (used under patristic); archive.org's Eastern Revelation commentaries (Andrew of Caesarea, Oecumenius) are modern copyrighted translations, lending-restricted. No public-domain Orthodox statement on Rev 17:1-18 found.
Reformed / Protestant
Matthew Henry says her principal seat and residence is upon the beast with seven heads and ten horns, namely Rome, the city on seven hills. He describes that city as infamous for idolatry, tyranny, and blasphemy.
Matthew Henry, An Exposition of the Old and New Testament (Matthew Henry's Commentary), vol. VI (Acts to Revelation) — On Revelation 17:3-5 ("Babylon the great" on the harlot's forehead)
Evangelical
John Wesley identifies Babylon as Rome. He says all things Revelation says of Babylon agree to Rome, and to Rome only.
John Wesley, John Wesley's Explanatory Notes on the New Testament — On Revelation 17:8-11