I just looked on the internet, and it seems like there's a LOT of speculation about it. Some people even think it's an allusion to what will happen in DMC5. That the poem hasn't happened yet.
What's even weirder is that the poem goes from ''a sword named after me'' to ''these prayers became our strength''. So the poem is first told by Sparda, then the villagers are the ones telling the story?
I came across some of those theories, too. And if the poem is intended to be broken up, switching the point of view from Sparda to the people, then maybe it will be easier to analyze.
But then again, that might be an error on their part, as well. I wonder if they will ever make a DMC5? I've heard rumours, but beyond that, no concrete evidence.