A.T. Robertson Commentary Revelation 17:5

"and upon her forehead a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF THE HARLOTS AND OF THE ABOMINATIONS OF THE EARTH." — Revelation 17:5 (ASV)

Upon her forehead a name written (επ το μετωπον αυτης ονομα γεγραμμενον). Roman harlots wore a label with their names on their brows (Seneca, Rhet. I. 2. 7; Juvenal VI. 122f.), and so here. In 19:16 Christ has a name on his garments and on his thigh, while in 14:1; 22:4 the redeemed have the name of God on their foreheads. There is undoubtedly a contrast between this woman here and the woman in chapter Re 12.

Mystery (μυστηριον). Either in apposition with ονομα or as part of the inscription on her forehead. In either case the meaning is the same, that the name Babylon is to be interpreted mystically or spiritually (cf. πνευματικως 11:8) for Rome.

The Mother of the Harlots and of the Abominations of the Earth (Hη Μητηρ των Πορνων κα των Βδελυγματων της Γης). The Metropolis of the Empire is the mother of harlotry and of the world's idolatries. Charles quotes Tacitus (Ann. XV. 44) about Rome as the city "quo cuncta undique atrocia aut pudenda confluunt celebranturque_."