John Milton wrote his magnum opus Paradise Lost to "justify the ways of God to men," as he states near the beginning of book I. On a smaller scale, the same could be said of the poem "On His Blindness," though here Milton seems to be trying to justify the ways of God to himself above all else.
Milton's eyesight was strained over time, and he went blind at the age of forty-three. Believing that his work was what allowed him to properly serve God, the speaker representing Milton in the poem wonders if this holy task is now impossible or if God expects his followers to continue working no matter what ailment might get in the way.
Milton's poem ultimately concludes that God does not require that everyone to undertake exceptional tasks in order to be of divine service. Instead, the task of any God-fearing person is to be patient with life and its many troubles and to have faith in God's will no matter how harsh those troubles might be. Milton describes this as standing and waiting:
His [God's] 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.
Servants stand at attention by a king generally to do whatever bidding he might require, be it slight or great. Milton now views himself from such a perspective, and in offering such an account, he justifies God's ways regarding the suffering and service of human beings.