Prior to that, "On His Blindness" was usually referred to by its opening line, "When I Consider How My Light Is Spent," but its subsequent title "On His Blindness" has been embraced over the last two and a half centuries.

In line 3, Milton refers to a "Talent" he can no longer apply toward labor he feels would be pleasing to God. This is an allusion to the parable of the talents in the Gospel of Matthew, chapter 25. In Jesus's lifetime, a talent was a weight of precious metal and therefore equivalent to money. In the parable, a man goes off on business, leaving different numbers of talents (that is, different values of money) to three servants. Two invest the money entrusted to them and earn profits for their master. The third merely buries the talent he was given, thereby earning no additional value. In Milton's poem, the speaker "buries his talent" by allowing a professional skill to go to waste. Unlike the servant in Jesus's illustration, however, the speaker has no choice but to do so. He's limited by a physical challenge— blindness (we should note that Milton's own blindness was already in full effect by the time the sonnet was published). Poetry is often assessed by how well it incorporates literary allusions and other forms of metaphor.

Milton's Gospel reference is a particularly evocative method of describing the speaker's physical limitations, casting them as an obstacle hindering his ability to please his God rather than merely an imposition to himself.

It is important to note that in line 8, the word "fondly" carries its now-outdated meaning of "unwisely," not the contemporary meaning of "affectionately." The speaker here asks God a foolish question: I want to please God, he insists, but will the Deity judge me harshly for being unable to use my full abilities? The reply, which the speaker hears as a whisper from personified "Patience" in his mind, reminds him that God is perfect unto Himself and requires no help whatsoever from human beings. He has the aid of thousands of angels at his beck and call. Thus, Patience reminds the speaker to serve God meekly, without worrying overmuch about the limits of mortal abilities. Patience's advice culminates in a now famous line: "They also serve who only stand and waite."

Some critics and commentators believe "On His Blindness" to be Milton's noblest sonnet. There is clearly an autobiographical element to the poem, and we empathize with Milton and his handicaps and resulting self-judgment. Most humans grow old. Our bodies grow infirm; we lose the vitality of youth and find ourselves unable to do some things we used to find important and pleasurable. It's easy to wonder if our value to the world around us has diminished, or even if God might be angrily punishing us for failing him somehow.

The universality of Milton's poem has endured for over three hundred years. It articulates his answer to the ever-present question of suffering. If God loves us, why does He allow our lives to be so difficult and short? Milton seems to imply that our struggles on earth are merely preparation to serve the Universal King most fully in the higher plane.

“On His Blindness” is written as a Petrarchan sonnet, with fourteen lines in two stanzas; the first, which has eight lines, is called the octave, and the second, of six lines, is the sestet. The poem also employs abba rhyme repeated once in the octave and cde repeated once in the sestet, which is typical of this sonnet form. The meter is iambic pentameter.

Milton uses figurative language to explore his theme of blindness, including the now well-known line “When I consider how my light is spent,” in which “light” is a metaphor for “sight.” Similarly, the darkness of the world offers a metaphor for the dangers of mortal life.

Allusion figures prominently in the devices he employs, especially references to lines and passages in the Bible. These allusions are especially suitable because Milton considered that he worked in service to God, but he also supported political causes that he believed were just. Notably, he refers to “Talent,” alluding to the New Testament Parable of the Talents, which comments on silver that a master gives a servant. He plays with the idea of talent—in his case, his writing, not a physical gift—as curtailed by his loss of sight. He evokes his concern that God will find his efforts wanting now that his blindness inhibits his ability to write. A further allusion is his use of “yoke,” also mentioned in the biblical book of Matthew; Milton applies the term to his responsibility to serve God, which he feels as a “mild,” rather than heavy, burden.

The end of the poem answers his initial query about the value of his service by concluding that his patient efforts have some value. The abrupt shift in language gives a quiet power to this conclusion. From an evocation of speed and space, with God’s power to move people quickly over all of land and sea, the poem concludes with an image of stillness. Rather than identifying himself with “I,” he implies his belonging to a larger group of patient, faithful believers: “They also serve who only stand and wait.”

"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.

"On His Blindness" by John Milton is written in the form of an Italian sonnet. It is an autobiographical poem written in the first person. It was written after Milton, a deeply religious writer, goes blind. 

The sonnet consists of two parts: an octave in which Milton laments his blindness, and a sestet in which he becomes reconciled to the blindness because he realizes God has willed it for a purpose.

The main crisis Milton experiences in the poem is not so much the blindness itself but the way the blindness interferes with his ability to write. He feels that his writing is doing God's work and his crisis derives from feeling that his blindness is a manifestation of God's rejection of him. He resolves this crisis by realizing God does not need or value us according to human standards, but rather according to our faith and obedience, as is conveyed in the lines:

God doth not need

Either man's work or his own gifts, who best

Bear his mild yoke, they serve him best.

I will get you started on this and you can do the rest.

As you know, Milton lost his sight and in this sonnet, he is lamenting its loss yet trying to console himself that his life still has purpose. Milton had a deep knowledge of Scripture (that is how he was able to write Paradise Lost), and in this poem, you can see the influence of his faith. While he is exploring his feelings about being blind, he seeks comfort through his faith. There is a lot of light/dark imagery:

 “my light is spent”

and

“ere half my days in this dark world and wide”.  

He states that he still has gifts and “talents” and that he can use them to serve “my Maker.” He admits that God does not need his or anyone’s help:

God doth not need either man’s work or his own gifts 

Nevertheless, God wants man to serve him. This Milton will do, and he says that he will wait on God to let him know the direction this service will take him, even though he may not be able to use his former gift of writing to serve God. When God closes one door, he opens the other. This is what Milton believes.

Look at the structure of the poem and see how he uses language to express this idea. There are lots of poetic devices and the word choices evoke a quiet, contemplative mood. Read more about Milton here on enotes.