Final answer:
In 'Dover Beach,' Matthew Arnold compares faith to a retreating sea, symbolizing the decline of religious belief.
Step-by-step explanation:
In Matthew Arnold's poem Dover Beach, faith is compared to a sea that was once full but is now retreating. The comparison is drawn in the third stanza, where Arnold depicts the Sea of Faith as being at its full tide and then notes its ebb. This metaphor introduces the concept of the waning of religious belief, illustrating how faith, once encompassing and sure, has receded like the ocean waves, leaving behind a world bereft of certainty and exposed to the harsh realities of life without the comfort of faith.