Faith is the pierless bridge
pierless bridge
A bridge without piers—no visible supports touching the ground. The paradox is the point: faith holds you up without anything underneath.
Too slender
Faith is invisible ('too slender for the eye'), yet the poem insists it's also made of steel. Dickinson wants both fragility and strength.
rocked in steel
'Rocked' like a cradle—maternal comfort. But the cradle is made of steel, not wood. Faith as both nurturing and unyielding.
could we presume
The subjunctive mood—'if we dared to assume.' The dash after 'what' creates suspense: she won't tell us what faith connects to.
vacillating feet
Latin *vacillare*: to waver, stagger. Our feet are unsteady, but the bridge is 'a first necessity'—we need it precisely because we're unstable.