I have always seen a strong Neoplatonic influence in that particular line of Keats'.
 
Beauty comes first. It is through beauty that the mind can rise to know Truth, and not vice versa. Beauty means Virtue, hence Beauty is the good -- the supreme good is Truth. Keats' Beauty is Plato's eros, guiding the mind to the discovery of the real world, the world of Ideas. This re-interpretation of Plato fits the Romantic idea of exotism and escape from reality.
 
I believe a good paraphrasis of that line would be, "Beauty leads to Truth, and Truth is made known by Beauty". Indeed, that is all we know on earth (for we can physically perceive Beauty) and all we need to know (because Truth is not immediately accessible in this lower world).  Hence, Beauty is a sort of hyponym to Truth.
 
This is just my opinion, anyway.
Sara-