The speaker of "The Black Heralds" is a first-person singular (we know because they say "I don't know!" all the time). The speaker seems to be trying for a universal identity—we don't know if it is masculine or feminine, it doesn't seem to have any racial or class markers. Also, the fact that there are no characters, really at all—just "man" as in "mankind" or (feminists please excuse us!) "humankind"—gives the poem and the speaker the feeling that it is timeless and placeless.
But the speaker is speaking to an audience that is familiar with Christianity, and the speaker seems to be someone who has fallen hard off the believers' wagon. Things like "the deep falls of the Christs of the soul" or the "pool of guilt" give off the feeling that this is a person who has lost their faith. They're extremely disappointed with the world and all of its trials and tribulations, and the speaker uses the poem to say so.