Milton wrote this sonnet blind. He asks, in anguish, how to serve God now that his light «is spent» before half his days are done, «that one Talent which is death to hide» lodged useless within him: «Doth God exact day-labour, light denied?»
But Patience answers before the complaint is out: God needs neither man's work nor his gifts; «who best / Bear his mild yoke, they serve him best». And the famous closing line sums it up: «They also serve who only stand and wait.» Faith understood as waiting.