Matthew Arnold's "Dover Beach" begins with the speaker looking out over a calm sea. By the end of the first stanza, the sound of the sea moving the pebbles on the beach has come to seem a melancholy one with an "eternal note of sadness."
In the third stanza, the speaker makes the symbolism of the sea explicit with his reference to "the Sea of Faith." By this, he means religious faith, specifically Christianity, which was once at high tide in Europe and gave meaning and purpose to people's lives. Now, he hears the "melancholy, long, withdrawing roar" of this meaning, and sees the "naked shingles of the world" exposed to the harshness of a universe without meaning.
"Dover Beach" was first published in 1867, but there is a critical consensus, based on Arnold's notes, that it was written much earlier, probably in 1851. This means that it cannot be a direct response to Darwin's On the Origin of Species, published in 1859, which is often taken to be the basis for Victorian expressions of nihilism and loss of Christian faith. Christianity was undermined by many factors in Victorian England, including geological research and the German Higher Criticism, which attacked the literal truth of the Bible. Arnold's poem shows that the "Sea of Faith" was receding in the minds of some highly educated Victorians before Darwin's explanation of evolution by natural selection was widely disseminated.