Apocalypse Revealed#860
860. Whose number is as the sand of the sea. This symbolizes the multitude of such people.
The multitude of such people is likened to the sand of the sea, because the sea symbolizes the external component of the church (nos. 402-404, 470), and the sand is something which serves no use in the sea except to compose the bottom.
Because the number of such people is so great, therefore the valley of their burial is called "The Multitude of Gog," and the name of the city where they are, "The Multitude" (Ezekiel 39:15-16).
