What is the main theme of "On First Looking into Chapman's Homer" by John Keats?

This poem narrates a memory of the speaker when he first
read a translation of The Illiad and the effect that this text had upon him. He
perceived the poetry so profoundly that this moment is compared to the excitement that
explorers feel when they look upon a tract of land that they have never seen before.
Note how Keats presents this simile:

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Then felt I like some watcher of the
skies

When a new planet swims into his
ken;

Or like stout Cortez when with eagle
eyes

He stared at the
Pacific...

Clearly then the
theme of this poem is the way that reading and literature allows us to travel in "the
realms of gold." For Romantics, poetry was the true adventure, and the best poetry
allowed you to enter new worlds of thrilling voyages and explorations. Thus the theme of
this excellent poem is that of adventure through imagination and the way that literature
is the key to enable us to engage in such flights of fancy and adventures, just as if we
were actual adventurers ourselves.