In Matthew Arnold's "Dover Beach," the poet speaks about a poet with the woman he loves who both look out the window of their room and react to the world around them, most especially the sea and what they see and hear along the shore.
In lines 21-23, Arnold writes:
The Sea of Faith
Was once, too, at the full, and round earth's shore
Lay like the folds of a bright girdle furl'd.
These three lines refer to religious faith—Christianity, specifically: that once it was everywhere, surrounding all things like a scarf and uplifting the world. But this is no longer the case. The sea seems to go on forever and be powerful in its own right. The narrator feels that Christianity also had this power once, and an endless reach—but now he feels differently.
Literary devices are considered first figurative language—descriptions that are not meant to be taken literally. Beyond this, it appears that the author has used both a metaphor and a simile in this segment (which are both examples of figurative language, and examples of imagery—descriptions that create an image or picture in the reader's mind—as well).
A metaphor compares two dissimilar things that share similar characteristics. In the following lines...
The Sea of Faith
Was once, too, at the full...
...we notice that the author is comparing faith (Christianity) to the sea. He writes that "faith" was once a powerful sea. ("At the full..." means "large.") Perhaps he conceives that both the sea and faith seem mighty and unconquerable. The sea and "faith" share the common characteristics of being powerful and everywhere—large.
The remaining portion of the segment you have presented reads:
...Lay like the folds of a bright girdle furl'd.
Here again, there is a comparison of the sea, this time compared to the folds of a "bright girdle furl'd." In this comparison, the word "like" is used, identifying this literary device as a simile. A simile also compares two dissimilar things that share similar characteristics, but uses "like" or "as" in the comparison. Here "faith" used to be something that was wrapped around people...the world.
A "girdle" is loosely defined as...
...anything that encircles, confines, or limits.
It can also refer to a belt or cord or sash--and "sash" brings to mind a scarf. The image the reader gets is one of something being encircled, and the poet notes that Christianity used to encircle the world, like the sea.