De libero arbitrio diatribe sive collatio
(literally Of free will: Discourses or Comparisons) is the Latin title of a polemical work written by Desiderius Erasmus of Rotterdam in 1524. It is commonly called The Freedom of the Will or On Free Will in English.

Background
 was nominally written to refute a specific teaching of Martin Luther, on the question of free will. Luther had become increasingly aggressive in his attacks on the Roman Catholic Church to well beyond irenical Erasmus' reformist agenda.

Luther published in 1520 his Latin Assertio omnium articulorum (itself a response to Pope Leo X's bull Exsurge domine that threatened Luther with excommunication) which included the statement God effects the evil deeds of the impious as part of the Wycliffian claim that everything happens by pure necessity, so denying free will.  Bishop John Fisher published a detailed response to the Latin version's arguments as Confutation of the Lutheran Assertion in 1523. (For the popular German version of this work, Luther sanitized his text for Article 36 to remove the arguments that "God is the cause not only of good deeds in man but also of sins, and that there is no natural power of the human will to direct man's actions either for good or for bad.")

Erasmus decided necessity/free will was a subject of core disagreement deserving a public airing, and strategized for several years with friends and correspondents on how to respond with proper moderation without making the situation worse for all, especially for the humanist reform agenda.. 

Erasmus' eventual irenical strategy had three prongs:

 first, a dialogue Inquisitio de fide to turn down the general heat and danger, which asked the question of whether Lutherans were heretics and, because they accepted the Creed, proposed that Lutherans were not;
 second, a small book On Free Will addressed as much to issues of limits of authority, discourse, biblical interpretation, as to free will;
 third, a small book De immensa misericordia dei (On the Immense Mercy of God), written ostensibly as a model sermon, published the same day as On Free Will and not mentioning Luther; it set up that God was not arbitrary, against the claims of predestination.

Terminology
 Synergism is the idea that adult salvation or justification involves some sort of co-operation (noting that the co- does not connote equality of the parties, God's grace always being in some way prior.) This is the view that Erasmus prefers in On Free Will.
 Monergism is the idea that God brings about an individual's salvation or justification regardless of their co-operation. This is the view associated with the early leaders of the Reformation such as Luther.   
 Semi-Pelagianism is the idea that the beginning of faith is a free choice, with grace supervening only later. 
 Pelagianism is the idea that humans have free will to achieve perfection

Content
A scholar has commented: "De Libero Arbitrio is clear in what it opposes, less so in what it affirms." Another holds "The most important and lasting legacy of Erasmus' theology was its nuance". Because of his irenical anti-Scholasticism, Erasmus attempted to argue without dogmatism, over-systematization, insult or appeal to Scholastic methods.

The conclusions Erasmus reached drew upon a large array of notable authorities, including, from the Patristic period, Origen, John Chrysostom, Ambrose, Jerome, and Augustine, in addition to many leading Scholastic authors, such as Thomas Aquinas and Duns Scotus. The content of Erasmus's works also engaged with later thought on the state of the question, including the perspectives of the via moderna school and of Lorenzo Valla, whose ideas he rejected.

Preface

Erasmus' thesis was not simply in favour of undogmatic synergism, but that Luther's assertive theology was not grounded and bounded adequately, as can be seen from the headings of the Preface: 

 Luther's supposed infallibility
 Objectivity and scepticism
 Having an open mind
 Difficulties in the scripture 
 Essence of Christian piety
 Man's limited capacity to know
 Unsuitability of Luther's teachings

Luther's response to these (ignoring the first point) had the headings: Assertions in Christianity; No liberty to be a sceptic; Clarity of scriptures; Crucial issue: Knowing free will; Foreknowledge of God; Tyrannny of Laws; the Christian's peace; Christian liberty; Spontenaity of necessitated acts; Grace and free will.

Erasmus asserted that over-definition of doctrine (whether by Church Councils or by Luther's assertions) historically leads to violence and more schism or heresy. The mentality and mechanisms of heretic-hunting were encouraged not relieved by adding to the articles of faith (such as requiring belief for or against free will), requiring terrors and threats. Erasmus' extremely tentative affirmation (of synergism) comes from these reflections: not only is any nasty disputation un-Christian, but the assertion of extra doctrines promotes, in effect, evil. Later commenters put it that Erasmus loved Peace more than Truth. 

He found no justification in consensus or history for Luther's idea on necessity, except for Manichaeus and John Wycliff. 

He suggested that his (and, implicitly, Luther's) own preferences might owe to personality more than to other sources. These were "red rags to a bull" for Luther.

Free Will
Erasmus adopted an unusual definition of Free Will: the ability of an individual to turn themselves to the things of God. So this included not only conversion but more general daily moments.

In his response, Luther split his definition of Free Will to cover on the one hand moral things—where he allowed free will to operate— and on the other hand conversion issues—where predestination was the proper explanation.

Synergism and Causation
Erasmus explains prevenient grace by the analogy of a pre-toddler, too weak to walk on his own yet. His parent shows the child an apple as an incentive, and supports the child as the child takes steps towards the apple. But the child could not have raised himself without the parent's lifting, nor seen the apple without the parent's showing, nor have stepped without the parent's support, nor grasped the apple unless the parent put it into his small hands. So the child  owes everything to the parent, yet the child has not done nothing. (s57)

Super-Biblical Evidence
Erasmus took his evidence not so much from one explicit Bible passage, but from the innumerable passages that command humans to do things. God, being neither mad nor cruel, would not command humans to do things that are completely impossible: believing or converting is one. These commands make no sense without free-will: the justice of God requires natural justice: humans cannot be held responsible if they have no choice. 

As far as God is concerned, Luther's view was that God can do anything (voluntarism), even logically impossible things (which appear to us as paradoxes), and that they are good because God did them (nominalism);  while Erasmus' view is that God really is good (realism) and nothing bad can be ascribed to him.

Foreknowledge and predestination
In part, the disputation between Erasmus and Luther came down to differences of opinion regarding the doctrines of divine justice and divine omniscience and omnipotence. While Luther and many of his fellow reformers prioritized the control and power which God held over creation, Erasmus prioritized the justice and liberality of God toward humankind.

Luther and other reformers proposed that humanity was stripped of free will by sin and that divine predestination ruled all activity within the mortal realm. They held that God was completely omniscient and omnipotent; that anything which happened had to be the result of God's explicit will, and that God's foreknowledge of events in fact brought the events into being.

Erasmus however argued that foreknowledge did not equal predestination. Instead, Erasmus compared God to an astronomer who knows that a solar eclipse is going to occur. The astronomer's foreknowledge does nothing to cause the eclipse—rather his knowledge of what is to come proceeds from an intimate familiarity with the workings of the cosmos. Erasmus held that, as the creator of both the cosmos and mankind, God was so intimately familiar with his creations that he was capable of perfectly predicting events which were to come, even if they were contrary to God's explicit will. He cited biblical examples of God offering prophetic warnings of impending disasters which were contingent on human repentance, as in the case of the prophet Jonah and the people of Nineveh.

Free will and the problem of evil

If humans had no free will, Erasmus argued, then God's commandments and warnings would be vain; and if sinful acts (and the calamities which followed them) were in fact the result of God's predestination, then that would make God a cruel tyrant who punished his creations for sins he had forced them to commit. Rather, Erasmus insisted, God had endowed humanity with free will, valued that trait in humans, and rewarded or punished them according to their own choices between good and evil. He argued that the vast majority of the biblical texts either implicitly or explicitly supported this view, and that divine grace was the means by which humans became aware of God, as well as the force which sustained and motivated humans as they sought of their own free will to follow God's laws.

Erasmus's conclusion
Erasmus ultimately concluded that God was capable of interfering in many things (human nature included) but chose not to do so; thus God could be said to be responsible for many things because he allowed them to occur (or not occur), without having been actively involved in them.

Because the issue came down to broadest biblical interpretation (i.e., Total Depravity, etc.), rather than dispute over individual passages or philosophy, Erasmus held that the safe approach was to favour the historical interpretation of the church—in this case synergism—over that of a novel individual.

Aftermath
Luther's response to Erasmus came a year later in 1525's On the Bondage of the Will, which Luther himself later considered one of his best pieces of theological writing. Other writers are repelled by it.

Hyperaspistes

In early 1526, Erasmus replied immediately with the first part of his two-volume Hyperaspistes; this first volume concerned biblical interpretation. The second, larger volume was a longer and more complex work which received comparatively little popular or scholarly awareness; the second volume concerned free will, with paragraph by paragraph rebuttal of Luther. Erasmus regarded Luther's doctrine of total depravity as an exaggeration, and noted that an inclination to evil did not exist in Jesus nor in  his mother. 

As with John Fisher's Confutation, Luther did not respond to the Hyperaspistes.

Catholic and Protestant
Erasmus and Luther's debate had great impact, with Catholic writers including Erasmus placing an increased emphasis on faith and grace, while many Protestants, notably Luther's second-in-command Philip Melancthon, and the later Arminians (such as Wesleyans) adopted aspects of Erasmus' view. Some historians have even said that "the spread of Lutheranism was checked by Luther’s antagonizing (of) Erasmus and the humanists."

The Sixth Session (1549), Chapter V of the Council of Trent defined a form of synergism similar to Erasmus'. In 1999 the Catholic Church and the Lutheran World Federation (later joined by many other Protestant denominations) made a Joint Declaration on the Doctrine of Justification on a common understanding of justification, concluding that the theological positions mutually anathematized at the time of the Reformation were not, in fact, held by the churches.

A 2017 survey of U.S. Protestants found that fewer than half accepted a view similar to Luther's sola fide, including in "white mainline churches"

Translations
Luther and Erasmus: Free Will and Salvation, translated and edited by E. Gordon Rupp, Philip S. Watson (Philadelphia, The Westminster Press, 1969)
The Battle over Free Will.  Edited, with notes, by Clarence H. Miller. Translated by Clarence H. Miller and Peter Macardle. (Hackett Publishing, 2012)
Discourse on Free Will by Ernst F. Winter (Continuum International Publishing, 2005)

References

1524 books
16th-century Christian texts
16th-century Latin books
Books by Desiderius Erasmus
Philosophy essays