Because in the OT mountains represent religions. Take for example Daniel 2:34-35: - 34 You watched while a stone was cut out without hands, which struck the image on its feet of iron and clay and broke them in pieces. 35 Then the iron, the clay, the bronze, the silver, and the gold were crushed together, and became like chaff from the summer threshing floors; the wind carried them away so that no trace of them was found. And the stone that struck the image became a great mountain and filled the whole earth.
Jay, I think it is kingdoms, not religions in Daniel 2, based on Daniel 2:39-40.
39 And after thee shall arise another
kingdom inferior to thee, and another third
kingdom of brass, which shall bear rule over all the earth.
40 And the fourth
kingdom shall be strong as iron: forasmuch as iron breaketh in pieces and subdueth all
things: and as iron that breaketh all these, shall it break in pieces and bruise.
And the stone cut out of the mountain in Nebuchadnezzar's dream is the kingdom of God.
44 And in the days of these kings shall
the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people,
but it shall break in pieces and consume all these kingdoms, and it shall stand for ever.
So, it is talking about kingdoms, not religions in Daniel 2.
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The seven mountains in Revelation 17: 9 is referring to the location of where the woman sits.