The 27 Churches

In Milton Plate 37 you may read:
"And these the names of the Twenty-seven Heavens & their Churches Adam, Seth, Enos, Cainan, Mahalaleel, Jared, Enoch, Methuselah, Lamech: these are Giants mighty Hermaphroditic
Noah, Shem, Arphaxad, Cainan the second, Salah, Heber, Peleg, Reu, Serug, Nahor, Terah, these are the Female-Males

A Male within a Female hid as in an Ark & Curtains, Abraham, Moses, Solomon, Paul, Constantine, Charlemaine Luther, these seven are the Male-Females, the Dragon Forms
Religion hid in War, a Dragon red & hidden Harlot"

You may find more detail on the twenty-seven Churches at Jerusalem Plate 75:
"And these the names of the Twenty-seven Heavens & their Churches Adam, Seth, Enos, Cainan, Mahalaleel, Jared, Enoch, .....
Noah, Shem, Arphaxad, Cainan the Second, Salah, Heber, Peleg, Reu, Serug, Nahor, Terah:....
(these are the pagan gods of Asia):
Abraham, Moses, Solomon, Paul, Constantine, Charlemaine, Luther. (these are the course of judaism and Christianity) ....these Seven are the Male Females: the Dragon Forms
The Female hid within a Male: thus Rahab is reveald Mystery Babylon the Great: the Abomination of Desolation Religion hid in War: a Dragon red, & hidden Harlot
But Jesus breaking thro' the Central Zones of Death & Hell Opens Eternity in Time & Space; triumphant in Mercy"

The twenty-seven Churches are encapsulated in the Whore of Babylon and her seven-headed beast (the double organization of Church and State):

"Thus are the Heavens formd by Los within the Mundane Shell
And where Luther ends Adam begins again in Eternal Circle To awake the Prisoners of Death, to bring Albion again With Luvah into light eternal, in his eternal day."

So what in the world can all this mean? I can't explain it completely, but here's a try: look especially at this line:
"And where Luther ends Adam begins again in Eternal Circle".

He's talking of course about the Circle of Destiny.

Blake has given these names to various ages or cultures found in the pre-biblical period, the two testaments, and the 2000 years after; Blake perceived a period of 6000 years in all (very much like some fundamental Christians who think the world was created in 4004 B.C).

Damon in A Blake Dictionary (pp 34-35) casts much light on the twenty seven churches: they all exist in Ulro.