How does Milton aim to serve God in "On His Blindness"?

When Milton ends with his famous line, "They also serve who only stand and wait," he is referring to the idea that God is like a king who can employ many servants to do whatever He wishes whenever He decides to use them.

His state
Is kingly; thousands at his bidding speed
And post o'er land and ocean without rest...

There are some servants in royal palaces who do nothing but stand at attention at their posts wearing livery and simply making themselves available, since this is what the king is employing them to do. Milton is suggesting that a person like himself is already doing exactly what God wants him to do, because everything is determined by God. Therefore Milton has no need to ask questions such as why he is blind or what God expects of him. All he has to do in order to serve God is to stand patiently and wait like a servant posted in a king's palace, who may or may not be sent on errands or told to perform services of one kind or another. In other words, Milton is expressing unquestioning faith in the will of God.

In "On His Blindness," the speaker, who represents Milton himself, begins by complaining that his blindness prevents him from serving God as he would wish with his talent. A personified figure of Patience answers him, saying that God is a better judge of such matters than he is, observing,

God doth not need
Either man’s work or his own gifts; who best
Bear his mild yoke, they serve him best.

How does "On His Blindness" reaffirm Milton's unwavering faith in God?

In his poem "On His Blindness," sometimes referred to as "When I consider how my light is spent" (the first line of the poem), John Milton reflects on his gradual loss of his sight and what it means for his work and his faith.

The poem begins with these lines:

WHEN I consider how my light is spent
E're half my days, in this dark world and wide,
And that one Talent which is death to hide,
Lodg'd with me useless, though my Soul more bent
To serve therewith my Maker, and present
My true account, least he returning chide (lines 1–6)

First, Milton laments that his blindness will make his "Talent . . . useless." He fears he will no longer be able to work as a poet since he will not be able to see to read or write. However, he also says the circumstances increase his faith, as his "Soul [is] more bent / to serve . . . my Maker." Milton feels both cheated—since he cannot use his gift if he is blind—and reaffirmed by his faith, because he knows that he will cleave to God in his time of weakness.

Though Milton briefly wonders if his blindness is some form of punishment, he quickly returns to his gratitude to God. He finishes the poem by writing:

God doth not need
Either man's work or his own gifts, who best
Bear his milde yoak, they serve him best, his State
Is Kingly. Thousands at his bidding speed
And post o're Land and Ocean without rest:
They also serve who only stand and waite. (lines 9–14)

He reflects that God does not need him to be a poet; instead, God only needs Milton's devotion. Milton lists ways people can serve God, and he apparently alludes to himself in the last line as one who can "only stand and waite." His blindness will mean he has to practice his faith more passively, unlike those who "at his bidding speed / And post o're Land and Ocean." However, he recognizes that his faith is still a form of service to God. Milton seems to feel that, even if he cannot write any more while blind, he can still fulfill his primary duty on Earth—which is to serve his "Maker."