Long withdrawing roar

The Sea of Faith
Was once, too, at the full, and round earth’s shore
Lay like the fields 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.

Don Cupitt in "Sea of Faith" writes thus:

Matthew Arnold published “Dover Beach” in 1867, in his last collection of poems. It expressed the sense, common at the time, that the ancient supernatural world of gods and spirits which had surrounded mankind since the first dawn of consciousness was at last inexorably slipping away … From now on thinking Christians would either be revisionists of some kind, or else be consciously in a dissenting minority.

Arnold himself was a revisionist, for after turning away from poetry he published in the 1870s four books which would now be called radical theology. But he was primarily a thinker. In the English manner, he was a mixture of poet and shopkeeper, combining an intuitive, imaginative mind with a sharp sense of social reality.