Relief is the first thing the speaker feels, and shame is the second. In the opening stanza, "darkness long has veil'd my mind" and then "smiling day once more appears". Only once the gloom has lifted does the speaker see "the folly of my doubts and fears". The poem is written from the far side of a bad stretch, and its first impulse is to scold: the speaker "upbraid[s]" a "wandering heart" and blushes at having entertained "one hard thought" of God. The title promises peace after a storm, but the poem quickly shows that this peace is something the speaker must keep winning back.
The central idea is a contrast between a God who never varies and a human heart that never stops swinging. Stanza three states it most clearly: "God is love, and changes not, / Nor knows the shadow of a turn." The phrase echoes a line in the Epistle of James (1:17) about God having no "shadow of turning". Here it works beautifully inside the poem's own imagery of light and dark. The speaker's moods are all weather and shadow, while God is the one thing in the poem that casts none.
What keeps the poem from sounding smug is how honestly it admits failure. The speaker says this truth is "easy to repeat", but when "faith is sharply tried" he is "a learner yet, / Unskilful, weak, and apt to slide." A schoolroom runs through the poem. The speaker asks to be "taught" what he is "so slow to learn", then calls himself a "learner". Even the word "tried" suggests a test. Any student who has understood a lesson in class and then fumbled it in the exam will recognise the gap between knowing something and being able to do it. The verb "slide" makes the failure physical, like a foot slipping on ice. It also makes it effortless, which is why it is so frightening.
Formally, the poem is a plain, steady piece of work. It has six quatrains of eight-syllable iambic lines, a pattern known as Long Metre, which suits hymn singing. The rhymes mostly alternate ABAB, with "mind" answering "find" and "tried" answering "slide". A few rhymes loosen, though. "Repeat" and "yet" barely chime, and "Thee" and "away" don't rhyme at all. Whether or not this was deliberate, the effect fits the subject, since the neat pattern frays exactly where the speaker admits he is unsteady. The language stays almost entirely in short, everyday words, so the emotion comes across as sincere rather than decorated.
The turn comes in stanza five with "But, O my Lord". Up to this point the speaker has been struggling by himself, and the rescue arrives in a single gesture: "one look from Thee / Subdues the disobedient will." The contrast between effort and ease is striking. Self-reproach gets nowhere, while one look from God is enough. The speaker then calls himself "Thy rebellious worm", an image of extreme self-abasement. The poem ends quietly with that rebellion being "still", a word that suggests both calm and stopped movement.
The last stanza balances itself almost like a scale: "Thou art as ready to forgive / As I am ready to repine." The parallel phrasing sets human grumbling against divine mercy as equal and opposite forces, with mercy winning. Then the speaker divides the spoils: God receives "all the praise", and "shame and self-abhorrence" are left as the speaker's share. Many modern readers will find that last phrase harsh, and it is worth asking whether the speaker is being humble or simply hard on himself.
William Cowper is remembered as a poet and hymn writer, and he lived with serious depression. The poem never says which darkness it means, so it is safest to read it on its own terms. Even so, readers who know about his struggles may hear extra weight in its descriptions of a mind veiled and then released. Its lasting interest lies in its refusal to pretend that faith, or any hard-won calm, stays fixed once it arrives. The poem is built as a cycle of relief, shame, prayer, relapse and rescue, and it leaves that cycle open. Many readers, religious or not, will recognise how it feels to understand a truth perfectly and still forget it the next time things go dark.