On her forehead was a name written, MYSTERY, BABYLON THE GREAT, MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH and she is that great city which reigns over the kings of the earth.

The great whore in Revelation, chapter 17, is a symbol of false religion and is referred to as a whore because she will do business with anyone and with whom the "kings" of the earth, over which she rules, have committed fornication.  Revelation 1:6 tells us that God's faithful are "kings" and priests unto him.  The inhabitants of the earth are made drunk with wine of the whore's fornication.  "I saw a woman sit on a scarlet colored beast": the whore and the beast are described as being scarlet which means filled with sin.

The great whore sits upon [controls] many waters.  "Many waters" is a symbol of a diversity of religious bodies around the world.  She is also described as being decked with gold, precious stones and pearls which tells us that there are some truths in Babylon.

The woman rides the "beast" which indicates guidance or control of the beast [civil government or political entity].

The most likely candidate for the being the great whore is the Rome where the Vatican is located. Although the Vatican is not a country and only occupies about 900 acres of land, many nations of the world extend diplomatic status to her which is usually only given to countries.  Thus the preposition that the Catholic Church is the woman of Revelation 17:18 persists.

The Catholic Church, at worst, is likely only one of many factions which make up the woman.  Hinduism, Buddhism, Islam, Freemasonry, and even Humanism as a religion make up a great part of the woman.  But the most dangerous to the Christian faith is the cancerous elements masquerading as Christianity.

These include, but are not limited to, Mormonism, the World Council of Churches, the National Council of Churches, Denominationalism or all factions claiming to be Christian whose origins are of the reformation during which they came out of the "Mother" Church, taking on the names of individuals or of a particular doctrine, and became harlots through compromise of the doctrines and principles of Christ.