In this poem, the speaker laments that the once unifying power in the world (religion, and Christianity in particular) is no longer relevant. The speaker seeks a substitute for this loss of unification. He looks for something that will fill the void that the loss of faith has left behind. He looks to his companion, thinking that maybe love for another will satisfy his empty feeling.
The speaker uses the metaphor of the sea to represent religion. The sea (oceans) literally connect all the continents and thereby, all the people of the world. This is a metaphor for religion. Christian faith, according to the speaker, once had the ability to unite people across oceans and continents. But in his modern age, religion no longer provides the answers he seeks. He feels no unity with humanity.
The Sea of FaithWas once, too, at the full, and round earth’s shoreLay like the folds of a bright girdle furled.But now I only hearIts melancholy, long, withdrawing roar,