While this poem is not, on the surface, easy to understand, a few things can be gleaned from a first reading. Someone sleeps on top of a mast, a cloud reflects on its own situation, and a gull looks into the sleeper's dream. What could this all be about?
As with all good poetry, there are many readings which would make sense. One of them is a questioning of the nature of belief -- or knowledge. Since the title of the poem is "The Unbeliever" we might infer that the sleeper is the unbeliever, and he doesn't believe, perhaps, that his unusual sleeping spot is strange or perilous. It would be hard to imagine a less comfortable or more dangerous place to sleep than on the top of a mast; yet the sleeper sleeps, and dreams.
The cloud and the gull, too, each believe that their sites are secure. The cloud, a most insubstantial and easily-blown entity, believes that it is "founded on marble pillars". The gull, blown to and fro by the winds, thinks the air is "like marble".
Bishop could be meditating on the epistemological (what is knowledge or belief? if the sleeper believes that his place is secure is it any less so?) or metaphysical (the temporal world may seem substantial, but it is as easily dissolved as a cloud). The poetic images are powerful, and do much to prompt a reader's thoughts in either direction -- or an entirely different one.