Could I have an explanation of the poem "Dover Beach"? What is the central theme of the poem?

In the first stanza, Arnold describes very beautifully a placid evening.  He seems to be looking out his window, watching the moon and listening to the beach.  But then it turns melancholy; he says that the waves lapping at the shore "bring the eternal note of sadness in."  In the next stanza he continues on the sad note, saying that Sophocles heard the same noise ages ago and " it brought/Into his mind the turbid ebb and flow/Of human misery".  From here, Arnold draws an analogy.  He says that the earth used to be full of Faith, just like the seas are full.  Then he says that unfortunately, "now I only hear/Its melancholy, long withdrawiing roar."  He states that faith has retreated, and it is disppearing from off the earth.

His states that in this world that "Hath really neither joy, nor love, nor light,/Nor certitude, nor peace, nor help for pain" the only thing that we can really do is "to be true to one another".  Overall, the theme of the poem is very sad, that faith and goodness are fading in the world, and evidence of it is all around.  So, we must be true, loving, and kind to one another.