The poem discusses the idea that humans are nostalgic. People in all ages look back to a better, more ideal past, a time which is imagined as more innocent and ordered than the chaotic world of today. Arnold's speaker ruminates about how once people felt solid and secure in their religious faith, unlike in the Victorian era, a time in which religious truths have been undermined by the theory of evolution and other scientific advances. (Ironically, we now tend to look back to the Victorian period as a time when religion was settled and robust.) The speaker writes:

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.

The poem also discusses the idea that humans try to make sense and meaning out of the world and see a reflection of themselves in it. He mentions that Sophocles, the ancient Greek playwright, thought he heard emotion in the sounds of the waves of the sea, "the turbid ebb and flow / Of human misery."