Though "When I consider how my life is spent" begins with religious doubt, it ends up expressing with considerable confidence Puritan religious doctrine. The Puritans not only had strong feelings about questions like the relationship between faith and works; they also had strict standards for the way churches should look. They preferred a bare, plain space for their worship. During the English Civil War, they destroyed much of the stained glass and icons that survived from the middle ages. In other words, their position was as much aesthetic as religious. As a devout Puritan and a poet, Milton is thus in a tricky position. His art relies on ornamental literary devices, yet those same devices may seem overly ornamental, out-of-keeping with the plain aesthetic he favored in religious matters.
In keeping with this, the poem's use of alliteration is fairly sparse: the speaker is not interested in dressing up his or her anxieties. Instead, the speaker presents them plainly, directly. However, there are important moments of alliteration—for instance the repeated /d/ and /w/ sounds in line 2 with “days, in this dark world and wide,” or the /p/ sound in line 8 with “patience, to prevent.”
In these cases, the alliteration links together two otherwise discrete ideas: patience, for instance, becomes associated with prevention. Because of the alliteration, it feels like the speaker is suggesting that prevention is patience’s job, part of its identity. Similarly, the width of the world—its overwhelming size—becomes, thanks to the alliteration, closely linked to the world itself. For this newly blind speaker, the world is defined by its enormity, its huge expanse. Thus although though the speaker does not use alliteration heavily, the use of alliteration the poem does underline its argument and suggests connections between concepts that might otherwise seem distant from each other.