Milton wrote his sonnet "On His Blindness" after he went completely blind in middle age. The sonnet is autobiographical and written in the first person. Since Milton was a deeply religious writer, he tried to understand his blindness within the context of his faith.
The octave of the sonnet laments Milton's loss of sight. As a writer, Milton strove to serve God by writing on various religious topics. His greatest work, Paradise Lost, tried to "justify the ways of God to men." In light of this, Milton feels his blindness prevents him from serving God, and wonders why God would deprive him of the means by which he expresses his faith.
In the sestet, Milton realizes the thoughts he had in the octave spring from his own egotism and false understanding of God. He realizes,
God doth not need
Either man's work or his own gifts.
Instead, what God desires of humans is faith and obedience, and Milton discovers that one can best serve God by subordinating one's own will to divine will and that service to God can take many different forms.