Satan, who was once one of God's principal...
Ironically, one of, if not the, most powerful speakers in Paradise Lost is the leader of the fallen angels, Satan. In fact, if he were a motivational speaker in the 21stC, he most likely would command very high prices for his speeches.
Satan, who was once one of God's principal angels, rebels against God because God has given precedence to Christ, and Satan's anger takes the form of a rebellion in which he convinces a substantial number of other angels to join him. His Latin name, Lucifer, means "Light giver," which places him perpetually in the perpetually ambiguous position of having an original name with the positive connotation of "knowledge giver" but who is also know as Satan, the adversary, the devil.
Satan's motivational skills are stunningly good, so good, in fact, that Milton ultimately takes them away in Book X, but at the beginning of the struggle between God and Satan, Satan is depicted as strong, proud (to a fault, of course), articulate, and intelligent.
After Satan and the angels have been defeated, "headlong themselves they threw/Down from the verge of Heaven," an act which, though desperate, is completely voluntary, an exercise of their free will. If the reader doubts whether the angels truly have free will, this should convince anyone that, at the very least, Satan and his fallen angels have either limited or complete free will.
Advancing the theme of free will, Satan declares to his troops: "What though the field be lost?/All is not lost; the inconquerable will. . . ." Later, Satan ties the concept of free will to the intellect by arguing that "the mind is its own place, and in itself/Can make a heaven of hell, a hell of heaven." This declaration, very dangerous in the context of the rebellion, basically says that each angel's intellect has the ability to create its own environment, in this case, either a heaven or a hell. Of course, astute readers might question Satan's logic here--if Satan and his fallen angels are doomed to lose the war with God and suffer eternal punishment, how complete is their "free will"?
As a good leader should, after the fall and while his troops are still somewhat discouraged and in disarray, he gives them perhaps the most powerful speech in the poem when he says that the battle may be lost but "the unconquerable will,/And study of revenge. . .And courage never to submit or yield,/And what is else not to be overcome?" In other words, their will and desire of revenge cannot be overcome, implying that they will be able to recover their original position in heaven. This speech has an electrifying effect on the fallen angels who "clashed on their sounding shields the din of war."
In essence, then, with Satan's declaration of the primacy of free will, he argues that the continued exercise of that will inevitably leads to the defeat of God. This speech, and the exhortation to fight, is Satan's declaration of independence and his commitment to wage perpetual war on God and his chosen beings.