So, I tried. It's not really the best I've ever written, nor am I totally happy with it, but this is the closest I can come at this point according to my limited understanding in discussing the difference between Augustine and Luther's concept of the Will in "about 1 page." It doesn't necessarily help that I have to reason from limited excerpts, though it does narrow the reading considerably.
Both read the same Scripture; both came to different conclusions. Augustine claimed Freewill; Luther asserted an Enslaved Will (On Grace and Free Will, Chapter 2 and following; On the Enslaved Will, Section IX).
Augustine argues that man must have free will because God commands him to choose between certain things, which choice is a function proper to the will (On Grace and Free Will, Chapter 3, 4, 5). To keep God’s law, free will must be assisted by grace and thus the achievement of the command is also due to grace (Chapters 6,7, 8, 9). God gives a man whose merit is evil a “good will” as grace. This “good will” is free and does good works with the help of more gratuitous grace (Chapters 12, 13) . These good works done with the help of grace by the good will (which was given by grace) are for a man “good merit” which God recompenses with Eternal Life (Chapter 14). Eternal life is by grace because God renders it according to works which proceed from God’s gift of faith; it is grace because it rewards the good life possible by God’s grace (Chapters 16, 17, 18, 19, 20). Free will is not taken away but works out its salvation aided by grace which is not remission of sin only, but fulfills the law, liberates the nature, removes the mastery of sin, and allows faith and endurance (Chapters 21, 27, 28). In this life a man receives “good for evil,” that is grace not according to merit, but “let us do good that in the future world we may receive good for good,” that is the crown of life in return for the merit of works performed by grace (Chapter 44). God is able to turn the will of man whithersoever he pleases, and men who are turned to believe in God had no previous entitlement to be turned (Chapters 41, 29, 43). God chose and loved us first and can we help but choose him afterward of our free will? Yet that choice has no merit unless God choose first (Chapter 38). God’s secret counsel is always righteous; when he hardens a man’s heart and turns him to doing evil it is in judgment of his own evil deeds (Chapters 42, 43, 44). To be sure, man hardens his own heart as well (Chapter 45). God turns him, but the man wills the hardening (Chapter 42). God for knows who will be ungodly and forsakes them in judgment on their deeds, withholding his grace (Chapter 45).
Luther disagrees. God does not for know by contingency (and so does not with hold grace from one because he forsees that he will do evil) but according to His unchangeable will (On the Enslaved Will, Section IX). God’s will belongs to His nature and as God’s nature is immutable, so is His Will. God forsees as He wills and wills as He forsees (OEW, Section IX). Man’s will is mutable, impotent and depraved (OEW, Section X). Salvation is the working of God alone - without co-operation of man’s will. Without the spirit of God, man does evil “spontaneously, and with a desirous willingness...which he cannot, by his own power, leave off, restrain, or change; but it goes on still desiring and craving (OEW, Section XXV).” Though human will cannot change its inclination – as it could were it free – God can change it; not at all by compulsion but so that it “desires and acts...responsively, from pure willingness, inclination and accord; so that it cannot be turned another way by anything contrary, nor be compelled or overcome even by the gates of hell; but it still goes on to desire, crave after, and love that which is good; even as before, it desired, craved after, and loved that which was evil (OEW, Section XXV).” The will is compared to a mount which cannot run to a particular rider. Satan and God contend for the saddle and the winner directs the beast. Rather than salvation being given to man by works, however instigated, supported, and fueled by ‘grace,’ God “promised to save me, not according to my working or manner of life, but according to His own grace and mercy” which is due solely to His own will. “In this way we please God not from the merit of our works but from the favour of his mercy promised unto us; and that, if we work less, or work badly, He does not impute it unto us, but as a Father, pardons and makes us better (OEW, Section CLXIV).”
Luther and Augustine’s disagreement goes beyond the “simple” question of whether a man has free will to the core question of how a man is justified. Both Augustine and Luther maintain that man is saved by God’s grace, but by that they mean two very different things. Augustine’s salvation by Grace consists in God giving man a good will, and helping that will by grace to do good works, and at last rewarding those good works with everlasting life, even though the man himself did not originate the good works (On Grace and Free Will, Chapter 20, etc). Luther’s salvation by grace casts works entirely aside asserting that faith in the word of God alone justifies. Salvation is entirely vicarious: the merit of Christ applied to man by faith, thus fulfilling the law (On the Freedom of a Christian, Part III). “For the word of God cannot be received and honoured by any works, but by faith alone” “and since it alone justifies, it is evident that by no outward work or labour can the inward man be at all justified, made free, and saved; and that no works whatever have any relation to him. And so, on the other hand, it is solely by impiety and incredulity of heart that he becomes guilty and a slave of sin, deserving condemnation, not by any outward sin or work (On the Freedom of a Christian, Part III).” Since salvation “depends on the working of God alone” who acts “according to his immutable, eternal, and infallible will,” human free will is “knocked flat and utterly shattered” and “those...who would assert ‘Free Will’ must either deny this thunderbolt, or pretend not to see it, or find some other way of dodging it (On the Enslaved Will, Section XXV, Section IX).” Luther would likely say that Augustine has done the latter.
I'd welcome feedback on whether I've got this correct or not. Or any other commentary. I felt like I was chasing Augustine around in circles for a while. And while I loved Luther's writing - it was so beautifully worded I wanted to either laugh or cry - I'm not sure I was able to nail him down in my writing. Partly due to the fact that I was working from excerpts and had to reference those excerpts.