Arnold's "Dover Beach" is an expression of sadness and uncertainty, born out of a fear of what will happen if the loss of religious faith he is experiencing should continue. The poem does not indicate, in its opening, what its theme will be—on the contrary, the picture it paints is a quaint and calming one, a literal description of a beach on a calm night:
The sea is calm tonight.
The tide is full, the moon lies fair
Upon the straits; on the French coast the light
Gleams and is gone; the cliffs of England stand,
Glimmering and vast, out in the tranquil bay.
In a sort of inversion of the usual "pathetic fallacy" trope, then, we will soon see that the natural world around the speaker is actually a complete contrast to his mental state. While he is standing in a beautiful place, awash with calm, he himself is full of mental turmoil and uncertainty. Arnold's setting and place help us to understand that mental turmoil is not necessarily driven by physical uncertainty or lack of security—England is, on the surface of things, as it has always been. It is only "the eternal note of sadness" which indicates how different things really are.
The Sea of Faith
Was once, too, at the full, and round earth’s shore
Lay like the folds of a bright girdle furled.
Like a girdle, the island's faith once kept it enclosed and protected; now, the sea seems only a miserable ribbon between England and France, transmitting the same note of "misery" which Sophocles once heard, many thousands of years ago.
In the end, the speaker can see little hope for his own future, other than to cling to his beloved, in whom he still seems to find some solace. He describes himself and his countrymen in vivid terms: as "ignorant armies," all fighting as if in the dark. Their ideologies "clash" with one another, but they do not have any light on the matter—there is no certain knowledge either of them can use against the other:
And we are here as on a darkling plain
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night.