Source: footnote to Sonnet XIX on page 393 of the 1993 Norton edition of Paradise Lost
That Milton drew inspiration from his Christian faith goes without saying, but I do not believe that this was his sole inspiration. For this reason I offer an alternative to cobalt's interpretation of Sonnet XIX . Though Milton was a true Christian, he was also an ambitious poet who was looking for a theme whose scale matched his talents. I believe this poem can be read as a discussion of this conflict.
The pivot upon which the reading of this poem hinges is the first two lines:
When I consider how my light is spent
Ere half my days in this dark world and wide
The first line is commonly read as a reference to his blindness which became complete in 1652. However, since John Milton was born in 1608, dating the poem after 1652 raises the problem of whether the poet really thought taht in his late forties he had yet to pass the halfway point of his. If the poem were dated at the beginning of the 1640s when Milton was approaching the halfway mark of the biblical "three score years and ten" then "Ere half my days" would ring truer. But then "how my light is spent" can no longer refer to his blindness, and so the whole poem opens up to re-interpretation
The opening lines can be read instead as the beginning of a complaint of his wasted potential. With "light" meaning his inspiration rather than his eyesight. This complaint continues with
And that one talent which is death to hide
Lodg'd with me useless,
It must be remembered that the 1640s found Milton devoting his energies to pamphleteering for the reformist cause during the ferment surrounding the English Civil War. His career had become that of a journalist - in truth a political and religous propagandist. His writings survive as evidence of his commitment, but would not have taxed his literary talent. The line "Doth God exact day-labour, light denied?" can be read as suggesting that whilst he recognised the public writings to be his duty, he got no satisfaction from them.
The rest of the poem then becomes a chiding of himself for being so unsatisfied with the role in which his god had cast him. The beginning of Patience's answer is a reminder of his belief that though works may be for the glory of god, ultimately salvation is only available through the grace of god,
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.
and the final lines a reminder of the insignificance of his abilities in comparison to what god can draw upon:
Is kingly; thousands at his bidding speed
And post o'er land and ocean without rest:
They also serve who only stand and wait."
On this reading of the poem it is not only about faith and submission, but also ego and ambition. If it was truly written in the 1640s then Milton had a long wait before he was to begin work on Paradise Lost in 1660, an epic that finally met the demands he made of himself and his god. Even without accepting the argument for dating this poem sooner than 1652, it still stands as testimony to the crisis of confidence Milton suffered, and to the poet's honesty in admitting his struggle to maintain his faith, in himself as much as god.