The speaker of this poem is someone capable of seeing things that no one else can see. He is a poet-prophet of sorts. While Europe was setting out to rebuild itself after the Great War had ended, this speaker is saying, "Wait a minute, not so fast. We need to look at what kind of world we’ve left ourselves with, and what it might mean for the future." Obviously, the speaker is deeply pessimistic. He’s also not afraid to use religious imagery, although he puts his own, weird spin on it. He can be thought of as trying to repeat the achievement of the Book of Revelation, which has all kinds of amazing, memorable symbols but is also vague and wild enough that no one could say what exactly it is supposed to mean.
In the first stanza he uses a bunch of metaphors to evaluate the present state of the world, and in the second he has a weird vision, followed by "darkness" and a rhetorical question, which amounts to a prophecy regarding the Second Coming. The first person appears only twice in the poem, but the prophetic voice feels very distinct and personal throughout. Like Yeats in real life, this speaker has an interest in the occult, as we can see from his reference to the Spiritus Mundi. Otherwise, he doesn’t use a lot of fancy language; but phrases like "mere anarchy" and "stony sleep" demonstrate that he doesn’t exactly talk like a regular Joe, either. He presents himself as a moral authority and feels comfortable making general pronouncements about the state of things, such as "the best lack all conviction, while the worst are full of passionate intensity." Above all, he wants to be scary, and, boy, does he succeed.