For John's first readers, “Babylon” most naturally evoked Rome, and it also pictures the wider biblical pattern of proud, violent, idolatrous empire. Christians have applied it to later systems, including the papacy, but those applications are neither certain nor exhaustive.
The Grace Answer
First, untangle the symbols, because the question blends three of them. There is the beast. There is Babylon the Great. And there is the woman who rides the beast. They are not identical. Revelation 17 shows the woman seated on the beast, and then the beast and its horns turn on her, hate her, and destroy her. Any reading that simply equals the woman with the beast has missed what the text does with them.
Now the historical anchor. For John's first readers, Babylon most naturally evoked Rome, while also picturing the wider biblical pattern of proud, violent, idolatrous empire. The seven mountains on which the woman sits, the line of kings, the imperial imagery, the persecution of the saints, the luxury and vast commerce, all of it fits the Rome that pressed on John's first readers. That connection is not idle speculation; interpreters saw it early, and they were not obviously wrong. The wider pattern, wealthy and violent and idolatrous empire demanding the allegiance owed to God, recurs through history.
Where certainty runs out
The recurring error is not identifying Rome as a central reference. It is the habit of crowning each new pope, emperor, president, or institution as the final fulfillment. Christians have applied Babylon and the beast to many later powers, the papacy among them. Some of those applications expose genuine corruption and are worth hearing. None of them should be preached as certain or exhaustive, and Grace Answers will not treat millions of ordinary Catholics as knowing agents of a satanic system because of the church they were raised in.
So John's pastoral instruction is not go hunt the beast but test the spirits, whether they are of God. The discernment he calls for is loyalty to the crucified and risen Lord and a refusal to hand your worship to any counterfeit, whether it wears a crown, a robe, or a flag. Hold interpretations of the papacy or any other candidate charitably; sincere believers land differently, and it need not divide the family. What is not up for debate is this: no earthly power owns you. You belong to the Lamb who was slain and lives forever, the One no empire could hold in a grave.