[…] But patience, to prevent That murmur, soon replies, "God doth not need Either man's work or his own gifts; who best
"Patience" to the rescue! Patience is personified as someone who can talk sense into the speaker. Patience is often personified in Christian art because of its role in helping one to achieve important virtues like courage and wisdom.
The speaker is about to "murmur" his foolish question about whether God would be so cruel as to make impossible demands of work, but then his patience steps in to stop him. The rest of the poem is the reply made by patience.
First, patience points out that God does not need anything. God is complete and perfect. He doesn't need work or talents ("gifts") of any kind.
Bear his mild yoke, they serve him best. […]
Patience now scores its second point in the rebuttal to the speaker. Patience argues that those people are the best servants of God who allow their fates to be linked with and controlled by God, as if they were wearing a yoke.
Essentially, this means accepting things as they come, especially suffering and misfortune.
A "yoke" is a wood frame that is placed around the necks of farm animals, like oxen, so that they can be directed.
Patience doesn't want to make God sound like a slave driver, so God's yoke is called "mild," or not-that-bad. It's not how much you have to show for your time on earth that counts, it's how you handle your submission to God.
[…] His 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."
The final point made by patience is that God is like a king, not a lord, so the "Parable of the Talents" does not strictly apply.
Lords need everyone on their estates to work for them; they usually don't have the resources to spend on keeping servants just to stand around and wait on them. Kings, on the other hand, have unlimited resources, especially if they control a "state" as large as the entire earth.
With His kingly status, God has plenty of minions to do His "bidding" by rushing from place to place – that is, doing things that require light and vision. It doesn't make a difference whether one more person fulfills the role or not.
But kings also have people who "wait" on them, who stand in a state of readiness until their action is needed.
To summarize, we believe that the sentence, "His state is kingly," is meant to contrast with the "lordly" state of the master of the Biblical parable in Matthew 25.
This being Milton, of course, "wait" can also have the meaning of waiting for something to happen, as in, "I waited for the bus."
What would the speaker be waiting for? The Second Coming of Jesus? The end of history? We don't know because the poem only suggests this meaning oh-so-vaguely.
The word "post" here just means "to travel quickly." That's why the mail is often referred to as the "post," because you're supposed to travel quickly to deliver it.
The poem ends with a vindication of the speaker's passivity, which has been forced on him by his blindness.