Martin Luther recognized in Erasmus’ challenge regarding the part free will plays in a person's salvation as a fundamental error in logic. The God hidden in sufferings and the cross was alien to Erasmus who failed, according to Luther, to properly distinguish between the hidden God and the revealed God. Erasmus did not distinguish between God or the will of God as preached, revealed, and worshipped and God as he is not preached, revealed, and worshipped. In consequence, Erasmus’ theology would not allow him to accept the radical bondage of the will Luther heard and experienced when God revealed himself in his word of the cross.
Although this way of speaking may have sounded radical to Erasmus, this was not a new teaching Luther came up with in 1525. Instead, "he came to realize the radical bondage of the will," writes Luther scholar, Gerhard Ebeling, "not in the face of the challenge of Erasmus as the advocate of the freedom of the will, but as soon as he comprehended the pure Gospel” (Luther: An Introduction to His Thought, 215).
Thus Luther’s response to De libero arbitrio should not be read as just a treatise filled with harsh judgments directed at Erasmus’ theological opinions. It also provides the material for determining the gospel against modern men of all eras. Erasmus’ opinions stood not only as representative of the Dutch pietism and Italian learning of the day, but as a kind of prototype of the modern evangelical Christian. That is, Erasmus wanted to be a theologian of grace. Erasmus laid out his argument for a theology of grace and free will in much the same way modern Protestants have done since the Enlightenment.
Many scholars have since read Luther’s response as a harsh judgment of Erasmus and a pessimistic attitude about human nature, but they've failed to appreciate Luther’s positive starting point. Luther began with the crucified God and allowed God to be God however uncomplimentary that may have been for Erasmus and those following the debate.
Failing to appreciate Luther’s starting point also means failing to appreciate his distinction between law and gospel. The deadly sting of the law in the denial of free will has generally been isolated from other aspects of Luther’s theology. And since it was precisely this that was so undeniably prominent in Luther’s writings, it cannot be bypassed or discussed in isolation from Luther’s theology without misrepresenting him.
For Luther, regardless of one’s standing corum hominibus (before men), the free will disappeared corum Deo (before God). That means that God’s revelation in Christ does not so much attack the atheist, or secularism, but the religious person who, in reality, is an atheist, and the religious society which, in reality, lives without God.
Through the law, as the alien word of God, the sinner is exposed, as Luther wrote, "not only bound, wretched, captive, sick, and dead, but in addition to his other miseries is afflicted, through the agency of Satan his prince, with this misery of blindness, so that he believes himself to be free, happy, unfettered, able, well, and alive” (LW 33:130).
Erasmus had committed a deadly error. By attempting to make a little room for free will, he was simultaneously attempting to replace the gospel with the law by passing off the doctrine of the bondage of the will as an unimportant academic argument. This was the essential matter in the debate for Luther.
He wrote about Erasmus that,
You alone...have attacked the real thing, that is, the essential issue. You have not wearied me with those extraneous issues about the Papacy, purgatory, indulgences and such like – trifles, rather than issues – in respect of which almost all to date have sought my blood...you, and you alone, have seen the hinge on which all turns, and aimed for the vital spot. For that I heartily thank you; for it is more gratifying to me to deal with this issue...
Erasmus had side-stepped the academic question of free will and struck out for the heart of the gospel: the matter of true theology and true knowledge of God. The division between the two men was in their distinguishing of the gospel word, or the will of God as preached, revealed, and worshipped; and the law - God as he is not preached, revealed, and worshipped. In the debate that followed this was their clearest point of divergence.