Matthew Arnold's poem "Dover Beach" is about the loss of faith and the continuity of sadness in the history of the world. He begins his poem listening to the sea, and then the sounds brings him back to ancient times in the second stanza. When Sophocles heard the sound of the sea, it brought "Into his mind the turbid ebb and flow / Of human misery" (lines 17-18). Arnold writes that "The Sea of Faith Was once, too, at the full" (lines 21-22). These lines mean that at the creation of the world, people were full of faith and love, just as the sea is full.
However, in the last stanza, Arnold remarks on the dismal state of modern affairs. He writes:
"Ah, love, let us be true / To one another! for the world, which seems / To lie before us like a land of dreams, / So various, so beautiful, so new, / Hath really neither joy, nor love, nor light, / Nor certitude, nor peace, nor help for pain; / And we are here as on a darkling plain / Swept with confused alarms of struggle and flight, / Where ignorant armies clash by night" (lines 29-37).
These lines mean that Arnold is calling for a return to true love and faith. The world, he writes, looks majestic and dream-like, but it really promises no happiness or love or any kind of reassurance. Instead, the world is like a plain on which armies are fighting in the dark. Only romantic love, his call to "be true," promises a release from the never-ending sadness of the world. Arnold starts the poem in the modern day, thinks back to ancient times, and then returns to the present. The continuity in the poem is sadness, meaning that humans are destined not to find their wishes fulfilled. Arnold is stating that humans are eternally sad and that only love can provide a respite.