"Not free, what proof could they have givn sincere..."

At the heart of John Milton's theodicy in Paradise Lost lies a free-will defense, and at the core of the free-will defense lies a defense of the free-will defense -- as we can see in this passage from Book 3:

. . . I made him just and right,
Sufficient to have stood, though free to fall.
Such I created all th' Ethereal Powers
And Spirits, both them who stood and them who faild;
Freely they stood who stood, and fell who fell.
Not free, what proof could they have givn sincere
Of true allegiance, constant Faith or Love,
Where onely what they needs must do, appeard,
Not what they would? what praise could they receive?
What pleasure I from such obedience paid,
When Will and Reason (Reason also is choice)
Useless and vain, of freedom both despoild,
Made passive both, had servd necessitie,
Not mee. (3.98-111)

Milton here has God the Father explaining why he gave human beings free will.

Why does Milton think that God needs to do that?

Because while the fact of free will might prove sufficient to bear the burden of explaining all of the world's evil -- if one factors in natural evil as resulting from the moral evil freely committed by, e.g., fallen angels (cf. Alvin Plantinga's free-will defense) -- what accounts for free will itself?

I mean, why should God grant free will if it might lead to evil choices. What accounts for God's choice to grant angels and humans free will?

Milton offers what he takes to be an overriding reason: Without free will, intellectual beings such as angels and humans would be serving necessity, not God.

In Milton's view, God is the ground of existence, and serving God is the aim of existence. Without free will, intellectual creatures cannot do this.

Thus, implies Milton, free will, though it enables free agents to choose evil, also enables genuine service to God and thereby justifies God's decision to grant freedom to intellectual creatures.

Serving necessity in place of God would perhaps constitute a variant of idolatry, though hardly a culpable one given the lack of free choice inherent in serving necessity.

But what is "necessity"? That is, what did it mean for Milton?

I don't have my Oxford English Dictionary at hand, so as a stopgap measure, let's check the Online Etymological Dictionary:

necessary 1340 (n.), c.1380 (adj.), from L. necessarius, from necesse "unavoidable, indispensable," originally "no backing away," from ne- "not" + cedere "to withdraw, go away, yield" (see cede). Necessary house "privy" is from 1609. Necessity (c.1374) is from O.Fr. necessité, from L. necessitatem (nom. necessitas) "compulsion, need for attention." Necessitate is first attested 1628.

So, "necessity" came into English by way of Old French from the Latin necessitatem, meaning "compulsion," and Milton would likely have been using "necessity" in the sense of a driving force.

I suppose that we can thank William the Conquerer for that compulsion.