The details of Luther's mid-1520s tussle with Erasmus over the issue of sin's impact on human freedom are generally well known. Luther responded to Erasmus's 1524 De libero arbitrio diatribe sive collatio with his own 1525 De servo arbitrio [On the Bondage of the Will]. Erasmus, deeply offended when the faux charity and grace he displayed in his work weren't reciprocated by the German monk, responded in turn with a decidedly less magnanimous two-part effort titled Hyperaspistes (1526/27). Luther never bothered answering this later work, largely because he felt that Erasmus had done a fine job of hanging himself in it--clearly evidencing to all the Pelagian tenor of his thought.
But Erasmus didn't completely fall off Luther's radar screen after 1527. In fact, as time went on, Luther became increasingly convinced that Erasmus was to blame for a considerable number of theological and social ills in Germany, not least the rising tide of Anabaptism. In 1534 Luther accordingly published an open letter to his friend Nicolaus von Amsdorf in which he expressed his distaste for Erasmus in no uncertain terms, calling him, for instance, a "palmerworm who [has] crept into the paradise of the Church, and filled every leaf with his maggots." Luther suggested that he himself had judged Erasmus too charitably in the past, finding him principally guilty of treating "the most sacred subjects" with too much "levity." He noted that his own effort to rouse Erasmus from his "snoring" -- presumably a reference to Luther's De servo arbitrio, which was addressed to Erasmus -- had only served to provoke Erasmus, like a deadly viper. Luther was now convinced, he confided to his friend (and anyone else who cared to tune in), that Erasmus's problem was "not simply levity, but [rather] malice and an entire ignorance of Christianity" (Henry Worsley, Life of Luther, 2:281).
Compared to some of the shots Luther fired in his lifetime, his remarks on Erasmus in 1534 seem rather mild. But they were strident enough to elicit regret from Philip Melanchthon over Luther's "petulance," a "petulance" Melanchthon was quick to chalk up to "old age" rather than innate temperament.
To be sure, Luther was quite capable of petulance, as any number of other exchanges might illustrate. But his concerns about Erasmus probably had more substance than Melanchthon realized.
Luther's comments about Erasmus were premised on a brief review of several of Erasmus's notable writings, with observation of some flaws. Thus, for instance, he took stock of a catechism Erasmus had written for children some years before, and noted how the Humanist scholar had failed significantly in his effort to articulate very basic Christian doctrine to those in need of sound, straightforward teaching. Indeed, Erasmus's catechism, in Luther's judgment, even served to undermine orthodox Trinitarianism by raising rather unfortunate (and decidedly unnecessary) questions about traditional teaching on the relationship of the divine persons. "Why in the Apostles' Creed," Erasmus asked early modernity's youngsters, "is the Father called God; the Son, not God but Lord; the Spirit neither God nor Lord, but only Holy?" No matter what answer followed, Luther noted, the question itself could only serve to engender doubt in tender minds about the full divinity of Son and Spirit.
In hindsight, Luther's concerns about Erasmus seem fairly well founded. The opening pages of Erasmus's Diatribe (for instance) do, it must be said, evidence a relative disinterest in, and disparaging of, fundamental doctrines such as the Trinity and Hypostatic Union in favor of (ostensibly) "clearer" biblical truths about how to behave one's self. Even Rome herself eventually turned on Erasmus, placing several of his works on the Index of Prohibited Books.
In short, if Luther's concerns and criticisms of Erasmus -- driven by Luther's profound sense of the need for clarity and precision in articulating the basic truths of our Christian faith; driven too by sensitivity to the significance of what a basic catechetical text doesn't say about its purported subject in addition to what it does say -- constituted petulance (as Melanchthon charged), perhaps more petulance is precisely what's needed in our own day.