Adam Clarke on Revelation 21:2

And I John - The writer of this book; whether the evangelist and apostle, or John the Ephesian presbyter, has been long doubted in the Church.

New Jerusalem - See the notes on Gal 4:24-27 (note). This doubtless means the Christian Church in a state of great prosperity and purity; but some think eternal blessedness is intended.

Coming down from God - It is a maxim of the ancient Jews that both the tabernacle, and the temple, and Jerusalem itself, came down from heaven. And in Midrash Hanaalem, Sohar Gen. fol. 69, col. 271, Rab. Jeremias said, "The holy blessed God shall renew the world, and build Jerusalem, and shall cause it to descend from heaven." Their opinion is, that there is a spiritual temple, a spiritual tabernacle, and a spiritual Jerusalem; and that none of these can be destroyed, because they subsist in their spiritual representatives. See Schoettgen.