The one thing you can rely on in this sonnet's speaker is that he can't be relied on. Donne opens with a complaint that sounds almost like a joke at his own expense: his "inconstancy" has "unnaturally" given birth to "a constant habit." He is faithfully unfaithful, steady only in his wobbling. The poem is a confession of spiritual moodiness, and its central idea lands in the last line with a sting. The days when he feels closest to God are not the calm ones but the ones when he "shake[s] with feare."
The first eight lines pile up contradictions like evidence at a trial. He changes his vows "when I would not," against his own wishes, so this is not carelessness but something nearer to helplessness. His contrition is "humorous," a word that in Donne's day meant ruled by shifting moods, and it is "as soon forgot" as his "profane love." That comparison is a quiet jab: his repentance is as shallow as a passing infatuation. The octave then closes on paired opposites: "cold and hot," "praying, as mute," "infinite, as none." The repeated "as … as" sounds like a list of measurements, but the measurements cancel each other out. He is everything and nothing at once, a riddle ("riddlingly distempered") he cannot solve.
The sestet swaps general confession for something like a three-day diary. "Yesterday" he dared not even look at heaven; "today" he is courting God with "flattering speeches"; "tomorrow" he will "quake with true fear of his rod." Courting and flattering belong to lovers and courtiers, and the choice of words hints at what troubles him: his prayer can turn into a performance meant to charm. Against this, the adjective "true" stands out. Fear is the only feeling in the sequence the speaker trusts. Then comes the central image, the "fantastic ague." An ague is a fever of alternating chills and sweats, so it echoes "cold and hot" and gives the poem a body. Faith becomes an illness that arrives in fits, and "fantastic," meaning capricious or imagined, suggests how little he can control it.
The form tightens around this chaos. The octave follows the Italian pattern of abba abba, though the a-rhymes are loose: "one" and "none" chime, but "devotion" matches "contrition" rather than "one." The sestet runs cddc ee and ends in an English-style couplet. That shape promises a problem followed by an answer, and the couplet's rhyme of "here" with "feare" delivers a satisfying click on the most important word. The answer, though, is a paradox rather than a cure. The rhythm is a flexible iambic pentameter that sounds like a man thinking aloud. The first line begins on a stressed, groaning "Oh," lines 2 and 3 spill over the line break at "begot / A constant habit," and line 9 stretches past ten syllables as if the speaker cannot stay inside the form. By contrast, the final line is made entirely of one-syllable words: "Those are my best days, when I shake with feare." Slow and flat, it reads like a verdict.
What makes the poem matter is its refusal to flatter its own speaker. Many devotional poems aim at certainty, but this one is about the lack of it. Donne turns the wit of paradox and oxymoron, so often used on love and argument, against his own soul. Beneath the cleverness is an uneasy honesty, because anyone who has made a resolution and dropped it by the weekend will recognise the "devout fits" that "come and go away."
The ending is also deliberately hard to read. It can feel like a bleak consolation: fear at least proves he is not fooling himself. It can also feel like despair, since the only reliable sign of faith is a shudder. The poem does not choose between these readings, and that open question is why it still unsettles readers.