Erasmus sought to find meaning behind the words of Scripture in order to make an ultimate claim. Luther, on the other hand, found the Gospel to be meaningless outside of Christ and his Cross.
For Luther, Erasmus’ Christ-less, Spirit-less theological conclusions demonstrated that behind his supposed humanistic optimism lay a profound despair and pessimism.
For Erasmus, it would be better for people in general to bear the disease of moralism and choice than to be cured of it by the preaching and teaching of God’s unconditional election of sinners in Christ.
When explaining that sinners were saved by grace alone Erasmus would not go so far as to say that the reception of God’s grace erased human responsibility.
Luther’s allies and opponents also would not allow him to put off responding to Erasmus indefinitely. They badgered him constantly to write a response.