Hello, Matititlane L
Welcome. I have a special penchant for that sonnet--it tugs at some of the unexamined corners of my heart. I have also finished a couple of master's papers on this one. It's a Keatsian gem.
Its broad theme is fear--fear of not finding true love and fear of untimely "ceasing to be." This is not exactly death in the conventional sense, per se. Keats had a gorgeous way of depicting death as a dissolving of sorts--a melting away from the world unseen--only to end up in a "low grass tomb." He was the proverbial stranger always in his poems--his identity constantly threatened by a quaking sense of despair that seemed to follow him everywhere--even if it was waiting silently around the corner from delight. This is the gist of the sonnet's overall theme. He, at the tender age of 22, mind you, was already feeling this perpetual ache (for love, literary recognition and later, death) and longing--it dogged his existence constantly. This sonnet marks that lurid feeling's beginning and derivation.
The first few lines denote his worry about leaving the world before he has the chance to write and be heard--"before my pen has gleaned my teeming brain." He then starts to talk about that "unreflecting love." He longs to feel and experience it before he is snatched away from the world (like so many of his family members by the time of this sonnet's composition--January of 1818). The "fair creature of an hour" is probably a reference to a woman he saw for only about 30 minutes at Vauxhaull--her loveliness was burned in his memory for years after his eyes locked with hers. He goes on to speak of that "nothingness" he so fears--that is characteristically Keats--he had a frightened bent toward that whole idea of "nothingness" greeting him when he left the world. He was not overtly Christian, nor was he religious. He is famous for saying, "Poetry is my Religion."
His tone is unabashedly pained and sad. He is expressing a deep seeded worry--a pathological fear of not living and experiencing all he longs to taste in life--Keats had experienced much loss as a boy--this sonnet is a reminder of sorts that, although he is young (when it was written), he must always be waiting for whatever happiness graces him at the moment to be spoiled by despair coming down the tracks...achingly beautiful, his life and plight--don't you agree?? I had written a paper about how this sonnet is a portention of sorts, as it almost eerily seems to predict his own untimely death...all of his work possesses that otherworldly quality--it's just one of the things that helps pronounce his genius.
Hope this shed an inkling of light...

"I am certain of nothing but the holiness of the heart's affections and the Truth of Imagination."