The speaker is facing a long, bleak night and finding no comfort in it. The opening image is striking: night is "pretty as a burnt church," beautiful but ruined, a place where something sacred has been destroyed. Yet the night still "preaches," and what it delivers sounds like Ecclesiastes, the biblical book that insists everything is vanity. The lines "the days of darkness shall be many" and "all that cometh is vanity" echo that voice, so the darkness feels less like a mood and more like a sermon on the emptiness of things.
The ending turns small and personal. The speaker takes "water / and a bitter little sin to bed," a quiet, almost domestic image after all the grand talk. The poem doesn't say what the sin is, and that silence is part of its effect. The mood is weary, dark and a little darkly humorous. The speaker doesn't fight the gloom. They accept it, carry their own guilt, and go to sleep.