Milton on libertarian freedom:

Granted, he doesn't call it that.

But his theodicy requires a free-will defense, so Milton puts one in the mouth of the Supreme Authority, God the Father, who tells us in Paradise Lost, Book 3.97-128 that he has created mankind with freedom so that mankind can freely remain loyal but that mankind will freely succumb to Satan's temptations and fall into sin and that mankind thus bears full responsibility for his freely chosen sin:

Whose [responsibility] but his own? ingrate, he had of mee
All he could have; I made him just and right,
Sufficient to have stood, though free to fall.
Such I created all th' Ethereal Powers [ 100 ]
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, [ 105 ]
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, [ 110 ]
Not mee. They therefore as to right belongd,
So were created, nor can justly accuse
Thir maker, or thir making, or thir Fate,
As if predestination over-rul'd
Thir will, dispos'd by absolute Decree [ 115 ]
Or high foreknowledge; they themselves decreed
Thir own revolt, not I: if I foreknew,
Foreknowledge had no influence on their fault,
Which had no less prov'd certain unforeknown.
So without least impulse or shadow of Fate, [ 120 ]
Or aught by me immutablie foreseen,
They trespass, Authors to themselves in all
Both what they judge and what they choose; for so
I formd them free, and free they must remain,
Till they enthrall themselves: I else must change [ 125 ]
Thir nature, and revoke the high Decree
Unchangeable, Eternal, which ordain'd
Thir freedom, they themselves ordain'd thir fall.

Milton describes a freedom that surely deserves the label libertarian, for God allows nothing to causally determine man's choices, for that would have man serving necessity rather than God, nor does God do anything to causally influence man's will in choosing, for that would place the onus for sin upon God. Thus, Milton's God insists:

I formd them free, and free they must remain,

Note, however, that Milton has God add the subordinate clause "Till they enthrall themselves." This is no offhand remark, though it sounds almost casual. Milton will need it as much later in his story as he needs libertarian freedom now. Mankind will enthrall himself to himself, precisely as the angel Abdiel informs the rebelling Lucifer:

Thy self not free, but to thy self enthrall'd; (PL 6.181)

Enslaved to his own self, mankind will require God's grace to lift him out of the pit into which he has fallen, thus initiating the entire Christian drama of redemption . . . which Milton only alludes to even in his follow-up poem, Paradise Regained.

But more on that another time.