He verbally assailed his friend, humanist and jurist Ulrich Zasius, for a doctrine proclaimed ten years before, and Erasmus's Annotationes in Novum Testamentum.

In his Obelisci, Eck attacked Luther's theses, which had been sent to him by Christoph von Scheurl, and accused him of promoting the "heresy of the Bohemian Brethren", fostering anarchy within the Church[3] and branded him a Hussite.

Eck did succeed in making Luther admit that there was some truth in the Hussite opinions and declare himself against the Pope, but this success only embittered his animosity against his opponents.

[3] Soon after his return to Ingolstadt, Eck attempted to persuade Elector Frederick of Saxony to have Luther's works burned in public, and during the year 1519 he published eight writings against the new movement.

Erfurt returned the proceedings of the meeting to the Saxon duke without signifying its approval, while Paris, after repeated urging, gave an ambiguous decision limited to "the doctrine of Luther so far as investigated".

Luther returned Eck's assaults with more than equal vehemence and about this time Philipp Melanchthon wrote to Œcolampadius that at Leipzig he had first become distinctly aware of the difference between what he considered to be true Christian theology and the scholasticism of the Aristotelian doctors.

Two satires, one by Œcolampadius and the other by Willibald Pirckheimer, angered Eck who pushed for the public burning of the entire literature in the market-place at Ingolstadt, an act from which he was restrained by his colleague Reuchlin.

[3] Johann Eck was more highly esteemed in Rome than in Germany,[3] where he induced the universities of Cologne and Louvain to condemn Luther's writings, but failed to enlist the German princes.

[1] In January 1520, he visited Italy at the invitation of Pope Leo X, to whom he presented his latest work De primate Petri adversus Ludderum (Ingolstadt, 1520) for which he was rewarded with the nomination to the office of papal protonotary, although his efforts to urge the Curia to decisive action against Luther were unsuccessful for some time.

[1] At Meissen, Brandenburg, and Merseburg, he succeeded in giving the papal measure due official publicity, but at Leipzig he was the object of the ridicule of the student body and was compelled to flee by night to Freiberg, where he was again prevented from proclaiming the bull.

On his return from his second visit he was the prime mover in the promulgation of the Bavarian religious edict of 1522, which practically established the senate of the University of Ingolstadt as a tribunal of the Inquisition.

[3] In addition to his inquisitorial duties, every year witnessed the publication of one or more writings against iconoclasm and in defense of the doctrines of the Mass, purgatory, and auricular confession.

[1] The affair ended decidedly in favor of Eck, who induced the authorities to enter on a course of active persecution of Zwingli and his followers (Conference of Baden).

[3] For the Diet of Augsburg, while still at Ingolstadt, Eck compiled what he considered to be 404 heretical propositions[6] from the writings of the reformers[3] as an aid to Charles V, Holy Roman Emperor.

He also was involved in the fruitless negotiations with the Protestant theologians, including Philipp Melanchthon, that took place at Augsburg; Eck with Wimpina and Cochlæus met the Lutherans in August.

[1] In January 1541 he renewed these efforts and succeeded in impressing Melanchthon as being prepared to give his assent to the main principles of the reformers, e.g. justification by faith; but at the diet of Regensburg in the spring and summer of 1541, he reasserted his opposition.