In The Odyssey and Dante's Inferno, the central themes are developed by introducing them in the opening of the narrative. In The Odyssey, two powerful themes are thus introduced: the intervention of the gods in humans' lives and the powerful attraction to and yearning for home. The first few stanzas/paragraphs (depending on the translation) summon the Muse and provide background. Immediately after that ensues a dialogue between the gods collected on Olympus, all except Neptune who is away in Ethiopia. The discussion addresses (1) Odysseus’ (also called Ulysses) longing for home and (2) the intervention of gods in his life (specifically Neptune's past negative intervention and the presently forthcoming positive intervention of all the gods assembled). These themes are then developed through the course of the narrative in the conflicts that arise and in the plot development (rising action, climax, falling action, resolution).
[Minerva said]: "You, sir, take no heed of this, and yet when Ulysses was before Troy did he not propitiate you with many a burnt sacrifice? ..."
[Jove said]: "Bear in mind, however, that Neptune is still furious with Ulysses for having blinded an eye of Polyphemus ... Still, let us lay our heads together and see how we can help him to return; ..."
[Minerva said]: "... if, then, the gods now mean that Ulysses should get home, we should first send Mercury ... to tell Calypso that we have made up our minds...."
In Dante's Inferno, the central themes are likewise introduced in the opening exposition. In fact, one is introduced in the very first stanzas:
Midway upon the journey of our life
I found myself within a forest dark,
For the straightforward pathway had been lost.
I cannot well repeat how there I entered,
So full was I of slumber at the moment
In which I had abandoned the true way.
These stanzas metaphorically describe how sins are introduced into a person's life, which is important since Dante's journey with Virgil through the levels of Hell is a journey through the sins, their causes, and the punishments of the inhabitants of the levels of Hell.
The related other most important theme is similarly metaphorically introduced in a metaphor likening the sun to God and God's redemption in a nearby stanza. It metaphorically describes the "good hope" attached to the sun and the stars that "with him were" when God, "Love Divine," set the beauty of the sky and world "in motion." These themes are developed as Dante travels ever deeper into Hell's circles--with Virgil providing most of the instruction about what Dante witnesses--where he learns the causes of sins (e.g., failure to make moral choices, deliberate choice of sin etc) and learns that confession of sin and quest for redemption would have caused God's mercy to be extended, like the rays of the sun, toward them, freeing them from sin and punishment.
The time was the beginning of the morning,
And up the sun was mounting with those stars
That with him were, what time the Love Divine
At first in motion set those beauteous things;
So were to me occasion of good hope,