Matthew Arnold signals his concern for the direction that society is taking; particularly the loss of religious faith in this poem. His language conveys this clearly.
One good example is the assonance in the line 'melancholy, long, withdrawing roar' which gives the poem a sombre feeling. Arnold also uses onomatopeia such as 'clash' which also signals a feeling of fear.
His use of a metaphor in which he describes the noise of the sea as containing 'The eternal note of sadness' also suggests a feeling of melancholy as the sea is like faith; retreating. He talks about the 'Sea of Faith' which was once 'full' and the poet fears that this will not be the case again. Arnold also describes the armies that he imagines as 'ignorant' which conveys his concern for what will replace religion-it does not appear to be positive.
Arnold uses the technique of listing all the things that the world is losing such as 'joy' 'love' 'peace' which underlines the feeling of sadness and loss.