The verb porneuo and noun porneia are prominent themes in Revelation. Two of the seven churches of Asia are warned about factions that commit acts of immorality (2:14, 20), and the great harlot Babylon fornicates with the kings of the earth (17:2; 18:3, 9). The noun form, porneia, appears seven times, perhaps suggesting a link with the days of creation: the creation has been infected with the evil of porneia

Together, the two words are used a total of 12 times, an Israel-specific number that fits neatly with the way harlot imagery is used in the Old Testament. Israel is repeatedly warned about prostituting themselves by following the gods and ways of the nations (Exodus 34:15-16; Leviticus 17:7; 20:5-6). Jerusalem is the prostitute city in Ezekiel 16 and Isaiah 1:23. The numerology is a clue that the harlotry of Revelation is the spiritual harlotry of the people of God.

To this we may add the five uses of porne, “harlot” (17:1, 5, 15, 16; 19:2). She is the mother of whores, with whom the kings of the land commit porneia and who shares her wine of porneia with the inhabitants of the land (17:2). She is the source of the 12fold porneia that fills the land. 

Those additional five uses of the porn- root might appear to spoil the numerology. Perhaps; but 17 uses is not insignificant. Augustine thought of 17 as the union of the 10 commandments of the old covenant with the sevenfold Spirit of the new. Again, perhaps: It would fit the logic of Revelation to see the harlot with her partners and her porneia as as counterfeit of the old covenant fulfilled in the new.

What is porneia? The harlot's behavior gives us the best clue. She is dressed as an Israelite high priest but rides on the back of a beast. She turns her paramours to beast worship, she herself being in bed with the beast. Porneia is idolatry, and specifically the idolatry of bestial Rome. The passion of this idolatry is the harlot's wine, which inflames those who drink with passion for the beast, and with passion to destroy the saints: The cup of the harlot is full of the blood of the saints. Porneia includes idolatry, imagined as sexual immorality, and the shedding of innocent blood, all three sins that pollute the land and expel its inhabitants.