There is a lot of imagery at play in the third stanza of “Dover Beach.” The extended metaphor in this stanza is religious belief being depicted in terms of a sea (“the Sea of Faith”), which laps at the shore of Earth. Arnold suggests that the “Sea of Faith” was once “at the full.” This image of a high tide signifies a time when people believed strongly in religion. Now, however, the speaker feels that the sea is “retreating,” and he observes that he can hear “its melancholy, long, withdrawing roar” as it pulls away, leaving the world unprotected by faith from harsh reality.
The speaker embellishes the extended metaphor about the Sea of Faith with an additional metaphor. He adds that in the past, the Sea of Faith “round earth’s shore / Lay like the folds of a bright girdle furled." This gives the impression that world was once wrapped up in and enclosed by the brightness and comfort of religion when that faith was still powerful and the tides of the sea were flowing inward rather than outward.
The idea that faith acts as something protective, like a garment, is intensified by the speaker’s comment that the “shingles,” or pebbles, “of the world” are left “naked” when it retreats. Without the girdle of the Sea of Faith, life is left stark and bare.