On the Freedom of the Will
De Libero Arbitrio Diatribe sive Collatio · Desiderius Erasmus, 1524
Not a Lutheran, and here on purpose. Erasmus was the most respected scholar in Europe, sympathetic to reform, and no defender of Rome's abuses — and he still thought Luther had gone too far in denying human beings any freedom in the matter of salvation.
Written carefully, reluctantly, and in a moderate tone, this treatise argues that Scripture leaves room for a small human contribution to salvation: not the power to save oneself, but a capacity to cooperate with grace once offered. It provoked Luther's furious, book-length answer, The Bondage of the Will, a year later.
You cannot fully understand what Luther was arguing against without reading what he was actually answering. This is that text.
Why there is no free text here
The standard scholarly English translation is E. Gordon Rupp's, published in the 1969 volume Luther and Erasmus: Free Will and Salvation (Westminster Press), and it remains in copyright. We could not verify a legitimate free public-domain English translation of Erasmus's own text, so we are not linking to one. If you find a copy worth adding, tell us.
Where to find it
University and public libraries carry the Rupp translation, usually paired with Luther's reply in the same volume. It is also excerpted at length within Luther's own Bondage of the Will, since Luther quotes Erasmus extensively before answering him section by section.
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The library tracks which works answer which. This is the conversation this text belongs to.