And we are here as on a darkling plain
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night
In the last lines of “Dover Beach”, Arnold points out the difference between the appearance and reality of the Victorian world. It looks new and beautiful like a land of dreams but in reality this world does not really have joy, love, light, peace, certitude or any help for pain. He describes the world as a dark plain which is becoming even darker as the time passes. The world has become a selfish, cynical, amoral, materialistic battlefield; there is much hatred and pain, but there is no guiding light. He compares the people struggling and running in their ambitions to the armies fighting at night, unknown of why and with whom they are fighting. Arnold makes an allusion to Greek historian Thucydides' account of the Battle of Epipolae (413 BC), in which Athenians fought an army of Syracusans at night. In the darkness, the combatants lashed out blindly at one another.
The last three lines of Arnold's poem "Dover Beach", express the author's bewilderment at the state of the world, which, instead of being filled with "joy...love...and light", is more like a battleground, where confusion and destruction reign. This elaborate simile, as well as the uses of alliteration and enjambment brings home the sense of loss of faith and its consequent horror.
In the third stanza of Mathew Arnold’s “Dover Beach”, the sea is turned into the "Sea of Faith" (l.21), which is a metaphor for the "ocean" of religious belief in the world—all of our faith put together. Arnold capitalizes this term and puts it all by itself at the top of the stanza to show its paramount importance. He is referring to a time (probably the Middle Ages) when religion could still be experienced without the doubt that the modern (Victorian) age brought about through Darwinism, the Industrial revolution, Imperialism, a crisis in religion, etc. Now, the 'Sea of Faith' and thus the certainty of religion withdraws itself from the human grasp and leaves only darkness behind.
Arnold illustrates this by using an image of clothes ('Kleidervergleich'). When that ocean of faith was at its height, it was like a "bright girdle" (that is, like a fancy belt) rolled up ("furled") around the world. He just uses a simile to compare his already-metaphorical ocean to a beautiful belt. The whole idea is meant to be a little ornate and complex, because what the speaker is describing, (the high tide of the sea of faith) is so mysterious and beautiful. When religion was still intact, the world was dressed ("like the folds of a bright girdle furled"). Now that this faith is gone, the world lies there stripped naked and bleak. The simile contrasts with "Vast edges drear/And naked shingles of the world."