The soul who spoke at the end of Canto V reveals himself: Justinian, the 6th-century Byzantine Emperor who codified Roman law into the Corpus Juris Civilis — the foundation of Western legal tradition. He is here in Mercury because his great works, though genuinely righteous, were accompanied by the desire for glory and fame. In the poem's moral economy, even this faint taint of self-seeking places a soul lower in Heaven, not outside it.
Justinian then delivers what is, in effect, Dante's entire philosophy of Roman history compressed into a single extraordinary canto. He traces the Roman Eagle from its origin in Aeneas's arrival in Italy through the kings, the republic, and the empire — citing battle after battle, ruler after ruler, in a sustained rhetorical flight. The Eagle is the instrument of divine providence: God chose Rome to be the vehicle through which the whole world would be prepared for the Incarnation. Caesar and Augustus were not merely conquerors; they were the means by which a unified, pacified world was made ready for "the true peace" — Christ's birth under Augustus's census.
This gives Justinian a precise framework for judging Dante's contemporaries: both the Ghibellines (who use the Eagle for their own political faction) and the Guelphs (who actively oppose it) are sinning against Providence. The Eagle belongs to no faction; it belongs to God's plan. To claim it for party use, or to obstruct it, is to work against the divine order of history. Justinian also mentions Romeo of Villeneuve, a humble counselor who served Raymond Berenger faithfully and well, was driven out by the envy of courtiers, and died in poverty — a brief portrait of loyal service unrewarded by the world but rewarded by Heaven.