ON HIS BLINDNESS , JOHN MILTON

When I consider how my light is spent
Ere half my days in this dark world and wide,
And that one talent which is death to hide
Lodg'd with me useless, though my soul more bent
To serve therewith my Maker, and present
My true account, lest he returning chide,
"Doth God exact day-labour, light denied?"
I fondly ask. But Patience, to prevent
That murmur, soon replies: "God doth not need
Either man's work or his own gifts: who best
Bear his mild yoke, they serve him best. His state
Is kingly; thousands at his bidding speed
And post o'er land and ocean without rest:
 They also serve who only stand and wait." 

The poem “on his blindness” of John Milton is organized into 14 lines. It has a structura of the tipical Petrarchan sonnet: two quatrains (octave) and two treizes (sestet). It has a rhyme scheme of ABBA ABBA CDE CDE. The octave explains the problem of the sonnet and the sestet expose a possible answer to the problem. 
The title “on his blindness” creates expectation because the reader doesn't know who is the blind.
However by the first line the reader notices throught the personal pronoun “I” the sonnet is about conditional existation of speaking voice. Indeed the octave is a consideration about  speaking voice's life. After he became blind he is afraid to hide his talent that is now useless, even if he is willing to serve his talent to God. So he expresses his frustration at being prevented by his disability from serving God as well as he desires to Lines 3-6 of the poem allude to the “Parable of the Talents”. In this parable an employer who is going away for a time gives his three servants money in proportion to their ability to increase its value. He distributes the money in talents: the first servant five talents of silver, the second servant two talents and the third servant only one talent. After the employer returns he asks for an accounting, the first servant reports that he doubled his talents to ten, the second that he soubled his to four and the third who ha only one talent hid it. So Both men receive promotions and the third servant is denounced by the employer for his laziness. The famous parable of the talents is the basis of the puritanism culture and the sonnet wants to comunicate the important code of values of the Puritanism.
Indeed the theme of the sonnet is the importance of ability of human beings useful to God. God judges humans on wheter they labor him to the best of their ability. The humans must avail of their talents and their abilities to allow social progress, indeed those who hide their talents, as the servant of the parable, is consider a sinner by God. 
God does not  really need man's work but is valued tha ability to tolerate whatever God asks faithfull without complaint. As the last line of the sonnet “they also serve who only stand and wait”.  Indeed the patience is considereted an important virtue.
The speaking voice learns that, rather than being an obstacle to his fulfillment of God’s work for him, his blindness is a part of that work, and that his achievement lies in living patiently with it.