In his lyric poem "Dover Beach," Matthew Arnold is lamenting the fact that the world, however beautiful it may seem, is ultimately devoid of certainty and peace, particularly as "The Sea of Faith" has now begun to recede from the shore. In the final stanza, Arnold describes humanity as "on a darkling plain" "where ignorant armies clash by night." Arnold feels that the solid basis of religious faith, upon which people once depended, has begun to ebb away. It is useful to understand the context for Arnold's poem: he published Dover Beach in 1867, less than a decade after Darwin's Origin of Species sparked debate globally over where life began and why we are here.
Once, Faith represented a "bright girdle" which supported and controlled society. Now, Arnold is saying, people can no longer have the "certitude" they once had, as competing points of view are being expressed, and the world of science is beginning to diverge from that of religion. In "Dover Beach," he presents this as a troubling turn of events, leaving people "swept with confused alarms," rather than safe in the understanding that faith in God is the only possibility, a truth from which "peace" could be derived.