The British king George III had recently died (in 1820) at an advanced age, suffering from blindness and dementia. Byron's literary and personal enemy Robert Southey had written a poem titled "The Vision of Judgment," in which the King is shown being welcomed into heaven. Byron, an anti-monarchist and liberal, thought that this deification of the king was nonsense. He created a spoof of Southey's poem with the purpose of attacking the political views that the king represented and ridiculing the sycophantic Southey. Byron hated Southey because he deserted the liberal Whig Party and became a Tory. As a member of the conservative establishment, Southey became Poet Laureate and "representative of all the race." Byron mocked him as such in the sarcastic dedication to Southey with which Don Juan begins.
In Byron's "The Vision of Judgment" a heavenly tribunal is convened in order to determine whether the deceased George III should be allowed to enter heaven or condemned to hell. The Archangel Michael acts as the king's advocate, while Satan argues that the king belongs in his domain, hell. Various witnesses are called to testify for and against, including the radical political reformer John Wilkes. The tribunal also hears from a writer who had used the pen name Junius. Multitudes of common people from various nations speak as well, and all could be viewed as victims of the poor judgment and obtuse leadership the king and his various administrations exercised on earth. St. Peter, the guardian of the celestial gate, presides over these arguments, but nothing is decided.
Finally the poet Southey appears and begins to read from his works. The entire assemblage of the supernatural host, both angel and devil, after hearing just a few lines of Southey's verse, find it so bad that they hold their ears and flee as if for their lives, abandoning the task of whether the king should be admitted to heaven. We are then told that in the midst of this confusion George III manages to slip unawares through the gate and into heaven, where he is heard "practicing the Hundredth Psalm."
Byron takes potshots throughout the poem against George III, Robert Southey, and the whole political and social establishment of his time. In inventing a comical, satiric episode about the question of justice in the afterlife Byron would seem to be attacking religion in general and Christianity specifically. His work was seen this way by the devout of his time. Still, the overall message is less of an anti-religious one than it is a commentary on hypocrisy.
Much of the poem is a kind of review of the last half-century of history. The wars and general mayhem of the revolutionary period in both America and Europe and the Napoleonic era, Byron seems to be saying, were not exactly George III's fault, and the man did not have some of the bad qualities such as marital infidelity of which monarchs are usually guilty. But at best, Byron is probably judging him as totally clueless. Like all kings, he was useless and obsolete. The problem is not specifically with this particular king, but with all kings. Byron takes issue with the way in which the world is governed, and the hypocrisy of the state and of government. He also takes issue with the hypocrisy of toadies (as Byron saw him) like Robert Southey.