How do the noise and the movement of "pebbles" mentioned in the first stanza relate to the idea of the loss of faith described in the rest of the poem?
Although "Dover Beach" by Matthew Arnold is overtly a poem about love, organized as a dramatic monologue addressed by the speaker to his beloved, thematically it is as much concerned with religion as romance. The dominant metaphor for religious faith in the poem is the sea. Just as the sea encircles Britain, and give the lands of the Mediterranean their character and sustenance, so the Sea of Faith encircles and protects humanity, cushioning us from an otherwise tragic, cruel, and meaningless life. Like the ocean, though, faith is dynamic rather than static.
In Arnold's period, Higher Criticism had challenged many of the common religious assumptions, and Arnold felt as if people were feeling lost and confused and battered by the changes and controversies in religion, tossed around like pebbles in the waves. He describes the withdrawing of the old religious certainties like the ebbing of the tide, leaving the pebbles exposed.