What is the theme of "On His Blindness" by John Milton?

Milton is asking himself what purpose he can have in life, now that he is completely blind. He was a deeply religious man and believed that the purpose of life was to serve God, which was what he had always tried to do. His way of serving God was to write poetry and essays on religious subjects or at least to write nothing but what he considered to be the truths that God would approve of. Milton's most famous work, of course, is his epic poem Paradise Lost, in which he said he wished to justify the ways of God to man. But being blind made it nearly impossible for him to write. That was "the one talent" he possessed. In his sonnet "On His Blindness" he asks whether God expected him to contribute anything to the world in spite of his severa handicap. He concludes by telling himself that God is all-powerful and does not need the services of any human being. His state is kingly. Humans who are patient and humble serve God best--those "who best bear His mild yoke." So Milton asssures himself that he is not sinning by failing to work for truth, justice, and religious understanding. The final beautiful iambic pentameter line of the sonnet summarizes the message of the whole poem: "They also serve who only stand and wait."

What causes Milton's lament in the sonnet "On His Blindness"?

The basic cause of Milton's lament is that he has always been a student, a reader and a writer, a literary man. Now that he is blind, he is unable to engage in the literary work which has given his entire life meaning. This would obviously be a terrible blow to a man of Milton's genius and dedication. What is impressive about the sonnet is the tone of patience which prevails throughout and which is summated in that final beautiful line of iambic pentameter: "They also serve who only stand and wait." He is not expressing frustration but patience, humility, and acceptance of the will of God. This is perhaps one of the most serious sonnets ever composed. It deals with an issue with which the reader can easily sympathize. How terrible it must be to be blind and unable to do the one thing you feel created to do! It is like Beethoven becoming totally deaf when his whole life was dedicated to his music and he was the foremost musical genius of his age. What Milton means by 

though my soul more bent
To serve therewith my Maker, and present
My true account...

is simply that he would like to be able to continue to exercise his great literary genius ("that one talent which is death to hide"). His soul is undoubtedly "more bent" to do so because, being blind, there is nothing else for him to do but sit and think thoughts he would like to be able to put down on paper and share with others. When something really tragic happens in one's personal life, it is natural to question, "Why does this have to happen to me?" Being a deeply religious man, Milton wonders why God would have given him the "one talent" and then denied him the power to use it. He does not allow his condition to weaken his religious faith.

What is the theme of Milton's sonnet "On His Blindness"?

Like most of Milton's work, this sonnet is an expression of his religious faith.

Milton was tormented by his blindness, and understandably so. Even in Paradise Lost he alludes to it, in a manner some critics have considered an inappropriate intrusion of a personal element into the otherwise lofty and austere character of his epic poem. Though the religious message in "On his Blindness" is sincere, it's hard to escape the impression that he is straining to convince himself that he can bear his affliction patiently. The opening lines clearly convey the agony he feels over being denied his sight. He seems to assert that his sorrow is due to his inability now to serve God, but there is an obvious, and honest, sense of resentment that emerges—an anger at God for having allowed this to happen to him. This in itself is a remarkable admission for a man as devout as Milton was. But he concludes by making the point that all God requires is that a person have faith. The phrase, "They also serve who only stand and wait" can serve, and has served, as a motto for anyone with a handicap. This is a theme Milton was ahead of his time in expressing: that all are equal before God regardless of their disabilities.