The couplet reads:
"And in the depts of my Dungeons
Closed the Fathers and the Sons."
Picture 12 inspires considerable thought: who are these people? we might suppose the old man is the inevitable Urizen or perhaps the God of Genesis 3. They seem to be in some enclosure like a prison cell. You might imagine that the two figures in the foreground could represent Adam and Eve, and perhaps the shape they were in right after the 'apple eating' incident.
Following that theme the remarkable thing is that God is in the same place as his two subjects. How could that be? It could never be that for a transcendent God, but Blake gave little or no thought to such a thing; In Marriage of Heaven and Hell, adapting the sobriquet of a devil he had this to say:
"The worship of God is: Honouring his gifts in other men, each according to his genius, and loving the greatest men best: those who envy or calumniate great men hate God; for there is no other God. "And in Jerusalem he wrote, "There is no other God than that God who is the intellectual fountain of Humanity". Many Quakers speak of "that of God" in you and me (called an immanent God), and that is pretty much Blake's God as well.
The theme and a very similar picture are found in Plate 16 of MHH where Blake states that "God only Acts & Is, in existing beings or Men."
These pictures describe the fate of those "caught in the toils of the world [who, poor men] fail to to develop or allow the expression of the divine spark within" (Digby 46).
Beneath the picture you might be able to read:
"Does thy God O Priest take such vengeance as this?"
Thy God O Priest! You might hark back to what Blake said about priests in MHH (Plate 11):
"Till a system was formed, which some took advantage of & enslav'd the vulgar by attempting to realize or abstract the mental deities from their objects: thus began Priesthood. Choosing forms of worship from poetic tales. And at length they pronounced that the Gods had orderd such things. Thus men forgot that All deities reside in the human breast",which happened to those trusting (in priests) souls seen in this picture.