Albert Barnes Commentary Revelation 17:9

"Here is the mind that hath wisdom. The seven heads are seven mountains, on which the woman sitteth:" — Revelation 17:9 (ASV)

And here is the mind which has wisdom. This is what requires wisdom to interpret; or, this is a case in which the mind that shows itself able to explain it will demonstrate true insight. This is similar to Revelation 13:18 (see the notes on that verse). Professor Stuart translates this as, "Here is a meaning which comprises wisdom." It is undoubtedly implied that the symbol might be understood—whether in John's time or afterward, he does not say—but it was a matter that could not be determined by ordinary minds, or without earnest application of the understanding.

The seven heads are seven mountains. This undoubtedly refers to Rome—the seven-hilled city—Septicollis Roma; see the notes on Revelation 12:3. On which the woman sits. The city is represented as a woman, in accordance with a common usage in the Scriptures; see the notes on Isaiah 1:8.