The poem does consider the ideas of faith and love which are often a feature of Victorian poems. The faith that Arnold refers to is faith in God which has come under attack because of recent scientific discoveries; for example those of Darwin. Religious faith is characterized as a sea that is receding with a 'long, withdrawing roar' leaving us to wonder what can replace it. The assonance in this line underscores the sadness that the poet feels.
For Arnold this takes the form of love between humans as he declares 'let us be true/To one another'. This seems to be all that is left as we are surrounded by the 'ignorant armies' in a time of increasing uncertainty.