A bridge without piers ought to collapse before anyone sets foot on it. Dickinson begins with that impossibility and calls it faith. In the first sentence of the poem, faith is a structure that carries us from "what We see" to "the Scene that We do not," from the visible world to whatever lies beyond it. The idea she builds toward is not that faith is comforting or beautiful, but that it is necessary, and necessary for a particular reason: we cannot be sure.
The first stanza makes its point partly through grammar. The sentence runs on across the line breaks, and then ends on "that We do not --", with the verb missing. We expect "see," but the line simply stops at a dash, and the sentence trails off into the unseen it describes. The bridge is "Too slender for the eye," so thin that it is nearly invisible, which is how a support with no piers has to look. Dickinson doesn't ask us to trust something solid. She asks us to trust something we can barely perceive.
In the second stanza the bridge becomes oddly physical. It carries the Soul "as bold / As it were rocked in Steel / With Arms of Steel at either side." Two ideas pull against each other here. "Rocked" suggests a cradle, something tender, while "Steel" suggests girders and heavy engineering. The bridge holds the soul like a child and like a load. But the phrase "as it were" quietly admits that the steel is only a comparison. Nothing actually surrounds the traveller. The word "bold" is also ambiguous, since it could describe the bridge's confidence or the manner of the crossing. Then the stanza ends on "behind the Veil." A veil hides something without removing it, and judging by the imagery, the far bank is whatever lies hidden from ordinary sight. The poem never names it.
The form is compact and quietly deliberate. Each stanza follows the same pattern of syllables, six, six, eight, six, so the third line always stretches a little further than the rest. Those long lines are "Unto the Scene that We do not --," "With Arms of Steel at either side --," and "To Our far, vacillating Feet." Each one reaches for something distant, which suits a poem about spanning a gap. The rhymes are the most interesting part. In the first stanza, "see" and "eye" barely match. In the second, "Steel" and "Veil" share only their closing sound. In the third, "be" and "Necessity" finally chime. It may be a coincidence, but the sound grows more settled as the argument approaches its conclusion.
That last stanza is the hardest to read, and it holds the poem's real claim. "To what, could We presume / The Bridge would cease to be / ... A first Necessity." Roughly, if we could take for granted what lies on the far side, the bridge would stop being essential. Faith matters only because certainty is not on offer. The speaker doesn't flatter herself or her readers, either. Our feet are "far, vacillating," and "vacillating" is a long, Latinate word among short plain ones, so it stands out as the poem's admission of weakness. The wavering feet are the reason the bridge is needed at all.
That honesty is what makes the poem worth reading. It would be easy to praise faith as a strong, certain thing, and the poem does give it strength, with its steel arms and its bold carrying. But the strength is placed next to doubt, not set against it. Faith here is a response to hesitation, not a cure for it. Dickinson's habits show up in miniature as well: the capitals that turn ordinary nouns into weighty ones, the dashes that leave a thought hanging over a gap, and a whole argument folded into twelve short lines. The final word, "Necessity," arrives like a plank laid down, plain and firm. The bridge may be almost invisible, but the poem insists we would not get far without it.