"The Sea of Faith
Was once, too, at the full, and round earth's shore
Lay like the folds of a bright girdle furled.
But now I only hear
Its melancholy, long, withdrawing roar,
Retreating, to the breath
Of the night-wind, down the vast edges drear
And naked shingles of the world.
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."
From these two stanzas of the poem "Dover Beach" there are references to the world that Montag inhabits, a world where there is no joy, or love or help for the pain, only emptiness.
There is a religious theme running through the book that the poem is used to remind Mildred and her friends about what is missing from life. This society has lost all emotional connection made through the passion and love of reading and sharing one's feelings. Mildred's friend gets upset because of the line that refers to armies clashing which reminds her about her husband.
Montag loves to read, and has remained faithful to the old way of life, like a religious devotion.
Arnold's great poem "Dover Beach" was written by a man who was, in addition to being a poet, an inspector of schools. Arnold believed very strongly in the necessity of education and shared culture as tools to hold back barbarism, and as ways to produce good and civilized people.
This relates to the themes of the novel because Bradbury's given us a world in which education, and particularly the printed world, is being both burned up and drowned in other activities. It's a world in which the anarchy that Arnold feared has won, at least for now, and the people don't know it. They are the ignorant armies clashing by night.