How does Arnold compare the modern world to a shingled beach in "Dover Beach"?

The comparison you mention is introduced in the third
stanza of this incredible poem, as the speaker takes the sound of the tide going out and
retreating from the land as a symbol of the way in which faith is declining in his land
during his time, similarly leaving the modern world naked and exposed, just like the
shingles on a beach. Note what this stanza says:

readability="23">

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.

In this stanza
therefore the speaker reflects on how the "Sea of Faith" was once full and protected his
land. Now, however, he hears the same "melancholy, long, withdrawing roar" as it
receeds, leaving the modern world exposed and vulnerable, with its "naked shingles"
showing. This comparison helps present Arnold's message that the decline in faith
actually leaves the modern world in a very dangerous and weak position, tranforming it
to a "darkling plain" where "ignorant armies crash by night."