In just a few lines, the speaker describes a suffering that feels heavy and hard to see. The "black mist of pain" is vague and all-surrounding, like something you can't grab hold of but can't escape either. The next line, "more than most believe," suggests the speaker feels the pain goes unrecognized, that other people underestimate how deep it runs.
The second stanza turns to religion. Instead of comfort, the speaker finds that "church after church seems / to hate me." The word "seems" matters here: it leaves room for doubt about whether the hostility is real or only felt, but the repetition of "church after church" suggests a long search that keeps ending in rejection. The title, "Theodicy," points to the old question of how suffering fits with a good God, which adds a sense of spiritual unease. The mood is bleak, lonely, and quietly bitter, made sharper by how short and plain the poem is.