"On His Blindness" uses the metaphor of light vs darkness to depict Milton's struggle with feelings of uselessness after going blind. The first line—"When I consider how my light is spent"—has the obvious meaning of considering how to spend one's daylight hours, but it also alludes to the purpose of human life itself: the "light" of the soul. If human beings are no more than beasts, they serve no greater purpose. Milton wrote in a deeply religious society, one that valued human life as superior. So, right away, he establishes in this poem that he is analyzing his spiritual significance in the world.
The question in line 7, "Doth God exact day-labour, light denied?" is another example of this layered meaning. In Milton's time, people worked by the light of day: there were no electric lights, and oil lamps and candles could be very expensive. In his blindness, Milton has been plunged into an eternal night, one with no work or productivity. But he realizes that this denial of light in the physical sense does not translate to the spiritual sense. Patience, a Christian value, quietly reminds him that God requires nothing from man. Often, an illness or disability, to a pious person, feels like a punishment from God. Certainly Milton, a writer who depended on his sight, would have felt this way. He struggles with the human need to have meaning in life, which stands in contrast to his spiritual beliefs: the common idea that God has a plan for everyone, even if it means pain and suffering.
The final lines show the resolution of Milton's crisis of faith and purpose. Patience reminds Milton that God has "thousands at his bidding speed/ And post o'er land and ocean without rest: / They also serve who only stand and wait." We know from lines 4 and 5 that Milton yearns to serve the Lord, and he feels that, in darkness, that ability has been taken from him. But God is the King of Kings, and he has thousands of servants who travel over land and sea at great speed. This is the life Milton desires, the one he values. But those "who only stand and wait" also serve God, and Milton seems to come to the conclusion that even with reduced abilities, the ability to serve God is not based on a physical state.