The painting is known as The Opening of the Fifth Seal, but that is not its subject.
The four seals are opened, and out ride the Four Horsemen of the Apocalypse, Death, War, Famine and Pestilence. When [the Lamb] opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained. 10 They called out in a loud voice, “How long, Sovereign Lord, holy and true, until you judge the inhabitants of the earth and avenge our blood?” 11 Then each of them was given a white robe, and they were told to wait a little longer, until the full number of their fellow servants, their brothers and sisters, were killed just as they had been.
I crib from Prospect magazine and Wikipedia. The damaged top half of the painting was removed in 1880, and Manuel B Cossio, who compiled a catalogue of El Greco’s works in 1908, suggested the title generally used now. Before, it was known as “Profane Love”. Wikipedia “suggests” that previously the upper half of the painting showed sacred love: that this is not known definitely shows the value the painting was held in- even now, if the title is a misconception. Yet it is credited as a major influence on Les Demoiselles dAvignon.
They do not look like the Blessed to me, even angry martyrs calling on God for vengeance. Those children do not look like cherubs, but souls falling into the Pit, and the man on the right could be standing on a rise, or falling. If the clothed man is a living human being and the unclothed people are resurrected dead, it makes sense to see him as St John, but his robe seems buffeted by winds, reminding me of the winds of the second circle of Dante’s Hell, where the lustful are blown about by an eternal storm. We look up at John from around his waist height, raising his hands to God in a way which fits the Great Day of God’s Wrath.
The red cloth dumped on the ground- with bodies underneath?- and the yellow pulled over the kneeling man’s face are not like the robes of the Blessed. Instead, blue yellow green red and white appear to me arrayed for pure composition rather than representation, proto-Mondrian.
If the upper half of the painting was a continuation of this image rather than a separate image, it could only be the sky, or the heavenly host. But John appears to be looking to our right, and slightly outwards from the painting, and I would have thought that the Lamb would command his attention, if visible. I also think the souls would be gazing enraptured or cowering away. I think the lost half is a related picture, rather than the same picture. So this would be the original composition rather than a fragment of it.
That little group of three in the yellow. The woman with her hand to her breast seems to be flirting with the man with his arm right out, looking up at him coquettishly, still, ignoring the storm. Perhaps in Hell we will still be able to use our feminine wiles. Or perhaps El Greco shared my interpretation, that the book is about the World before the Judgment, and living people suffer by death, war, famine and pestilence as we always have.