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).
Apocalypse Revealed#195
195. "'Which is coming down out of heaven from My God.'" This symbolizes doctrine that will come from the Lord's Divine truth, such as it is in heaven.
Since "My God" symbolizes Divine truth (no. 193), it follows, when it is said of the Lord and of the doctrine of the New Church, that coming down out of heaven from My God symbolizes doctrine that will come from the Lord's Divine truth such as it is in heaven.