In Jeremiah 2:10, this signifies gentiles in simple good, and who live naturally in truth and good according to their religious instruction. (Apocalypse Explained 406[14])

878. And there was no more sea. This symbolically means that the outer part of the heaven composed of Christians since the church was first established was likewise dispersed, after those who were written in the Lord's Book of Life were liberated from there and saved.

A sea symbolizes the external component of heaven and the church, where the simple are, who think naturally and not much spiritually about matters connected with the church. A heaven inhabited by these is called external, as may be seen in nos. 238, 398 at the end, 403, 404, 470, 567, 659, 661. Here the sea means the outer part of the heaven composed of Christians from the first establishment of the church. However, the inner part of the heaven of Christians was not fully formed by the Lord until shortly before the Last Judgment, and also after it, as may be seen from chapters 14 and 15, which have that heaven as their subject, and from chapter 20:4, 5, too. See the exposition in those places.

The inner part of that heaven was not fully formed sooner because the dragon and its two beasts held sway in the world of spirits, and they burned with a desire to lead astray whomever they could. It would have been dangerous, therefore, to gather together those Christians into a heaven beforehand.

The separation of the good from adherents of the dragon and the damnation of the latter, followed by their finally being cast down into hell, is the subject in many places, and lastly in chapter 19:20, and chapter 20:10. And after that we are told that "the sea gave up the dead who were in it" (chapter 20:13), which means that the external and natural people of the church were called together for judgment, as may be seen in no. 869 above, and that those who were written in the Lord's Book of Life were then liberated and saved, as indicated also in the same number. This is the sea that is meant here.

[2] We are also told elsewhere regarding the New Christian Heaven that the outer part of the heaven of Christians extended to a sea of glass mixed with fire (Revelation 15:2), and the sea there also symbolizes the outer part of the heaven of Christians. See the exposition in nos. 659-661.

It can be seen from this that there being no more sea means, symbolically, that the outer part of the heaven composed of Christians since the church was first established was likewise dispersed, after those who were written in the Lord's Book of Life were liberated from there and saved.

Regarding the outer part of the heaven composed of Christians from the first establishment of the church, I have been granted to learn many things - too many, however, to present them here. I will say only that the previous heavens which passed away at the time of the Last Judgment were permitted for the sake of those Christians who were in that outer part of heaven or sea, because they were conjoined by external concerns and not by internal ones - on which subject something may be seen in no. 398 above.

The heaven where external people of the church dwell is called a sea because their dwelling place in the spiritual world appears from a distance as being in a sea. For celestial angels, or angels of the highest heaven, dwell as though in an ethereal atmosphere. Spiritual angels, or angels of the intermediate heaven, dwell as though in an airy atmosphere. And spiritually natural angels, or angels of the lowest heaven, dwell as though in a watery atmosphere, which, as we said, from a distance looks like a sea.

For this reason the outer part of heaven is meant by the sea also in many other places in the Word.