Herbert opens with a small, almost cinematic scene: a traveller comes to a dark, shady grove, does not go in, and lifts his eyes to the sky above it. The comparison drives the whole poem. When the speaker looks at his sins, he does not stare at them or sink into guilt. His eyes "remove / More backward still" to the water of baptism. The direction is odd and rewarding. The water is both behind him, at the start of his Christian life, and "above the heav'ns," so looking back and looking up turn out to be the same movement.
That water is tied to a specific image, the "pierced side" of the "dear Redeemer." The wound and the baptismal font become one stream. The word "dear" is quietly tender, and the apostrophe that follows, "O blessed streams!", turns the poem from description into address. Herbert then speaks directly to the water as if it were a person who could be thanked or held to account.
The grove does not vanish after the first quatrain. It comes back as the sins "growing thick and wide," a phrase that makes wrongdoing sound like undergrowth. The water has two possible jobs: it can "prevent / And stop" the growth, or "give tears to drown them, as they grow." In Herbert's day "prevent" could also mean "go before," so the water may arrive ahead of the sins as well as block them. The second option is stranger and more moving, because drowning a plant is a clumsy image. It suggests a faith that will not pretend sin does not happen. Baptism either holds sin back or supplies the repentance that overwhelms it.
From line 10 the imagery shifts from landscape to something close to accounting and medicine. "Redemption measures all my time" sounds like a surveyor or a clerk. Then comes the "plaster" spread "equal to the crime," a bandage cut to the size of the wound. The rhyme of "time" and "crime" links the two ideas: every stretch of life, however guilty, has its remedy already measured. Herbert is not saying that sin is small. He is saying that grace is proportionate to it, which is more reassuring than a vague promise of forgiveness.
The last four lines turn to names and credit. The water "taught the book of life my name," so baptism is imagined as an enrolment, a first entry in a ledger. Any later sin can only "miscall" the speaker, a good word for it, since it means both to call by the wrong name and to slander. Sin, in this reading, is a false description of who he is. The closing phrase, "Your first acquaintance might discredit all," borrows the language of testimony and trust. Baptism is the earliest witness, and its word outweighs every accusation that comes after.
The form supports this confidence. The poem has fourteen lines and is built on a sonnet pattern in regular iambic pentameter, with a rhyme scheme running roughly abab cdcd effe gg. The fifth and seventh lines, "rest" and "prevent," only half-chime, and that looseness may suit a poem about a messy subject being kept in order. Then the closing couplet, "miscall" and "all," lands with a flat, legal finality. The sentences are plain, and the one long sentence of the opening simile resolves itself neatly in "So when I view my sins."
The poem matters because of the way it makes a theological idea feel practical. Baptism is usually thought of as a single past event, but here it reaches forward over the speaker's whole life. Herbert does this with homely comparisons, a grove, a bandage, a name written in a book, rather than abstraction. As a priest of the Church of England, he would have known the sacrament from the inside, though the poem itself never says so. What it offers is a way of looking, which is to glance at the dark grove honestly and then lift your eyes.