John of Patmos and William Blake were both visionary poets. Their minds were open to visionary messages from the Divine Inspiration. They communicated through describing the visual images they received translated into intellectual forms which their audience may understand. The forces which would build God's Kingdom and those which would destroy it waged battles in the minds of the two men. The great red dragon, and the woman clothed with the sun, were antagonsits whose contrasting images could demonstrate the nature of the opposed forces.
The Great Red Dragon and
the Woman Clothed with the Sun
From the description in the Brooklyn Museum web page:
"Inscriptions: Inscribed above the image: 'A Woman clothed with the sun, & the moon under her feet, and/upon her head a crown of twelve stars; and behold a great red dragon also.' Inscribed below the image at right: 'Revns:ch:12th: v 4th:' Inscribed below the image: 'And the tail of the great red dragon drew the third part of the stars of/heaven, and did cast them to the earth. And the dragon stood before the/woman which was ready to be delivered for to devour her child as soon as it was born.'"
 And there appeared a great wonder in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars:
 And she being with child cried, travailing in birth, and pained to be delivered.
 And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.
 And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
 And she brought forth a man child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to his throne.
 And the woman fled into the wilderness, where she hath a place prepared of God, that they should feed her there a thousand two hundred and threescore days.
 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
 And prevailed not; neither was their place found any more in heaven.
 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
 And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.
 And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.
 Therefore rejoice, ye heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time.
 And when the dragon saw that he was cast unto the earth, he persecuted the woman which brought forth the man child.
 And to the woman were given two wings of a great eagle, that she might fly into the wilderness, into her place, where she is nourished for a time, and times, and half a time, from the face of the serpent.
 And the serpent cast out of his mouth water as a flood after the woman, that he might cause her to be carried away of the flood.
 And the earth helped the woman, and the earth opened her mouth, and swallowed up the flood which the dragon cast out of his mouth.
 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.
Milton, Plate 39 ,(E 141) "So Milton Labourd in Chasms of the Mundane Shell, tho here before My Cottage midst the Starry Seven, where the Virgin Ololon Stood trembling in the Porch: loud Satan thunderd on the stormy Sea Circling Albions Cliffs in which the Four-fold World resides Tho seen in fallacy outside: a fallacy of Satans Churches PLATE 40  Before Ololon Milton stood & percievd the Eternal Form Of that mild Vision; wondrous were their acts by me unknown Except remotely; and I heard Ololon say to Milton I see thee strive upon the Brooks of Arnon. there a dread And awful Man I see, oercoverd with the mantle of years. I behold Los & Urizen. I behold Orc & Tharmas; The Four Zoa's of Albion & thy Spirit with them striving In Self annihilation giving thy life to thy enemies Are those who contemn Religion & seek to annihilate it Become in their Femin[in]e portions the causes & promoters Of these Religions, how is this thing? this Newtonian Phantasm This Voltaire & Rousseau: this Hume & Gibbon & Bolingbroke This Natural Religion! this impossible absurdity Is Ololon the cause of this? O where shall I hide my face These tears fall for the little-ones: the Children of Jerusalem Lest they be annihilated in thy annihilation. No sooner she had spoke but Rahab Babylon appeard Eastward upon the Paved work across Europe & Asia Glorious as the midday Sun in Satans bosom glowing: A Female hidden in a Male, Religion hidden in War Namd Moral Virtue; cruel two-fold Monster shining bright A Dragon red & hidden Harlot which John in Patmos saw And all beneath the Nations innumerable of Ulro Appeard, the Seven Kingdoms of Canaan & Five Baalim Of Philistea. into Twelve divided, calld after the Names Of Israel: as they are in Eden. Mountain. River & Plain City & sandy Desart intermingled beyond mortal ken But turning toward Ololon in terrible majesty Milton Replied. Obey thou the Words of the Inspired Man All that can be annihilated must be annihilated"
Urizen remembers his own fall.