Religious division continued to plague Charles’ empire, but the threat of the Turkish armies on the eastern border of his lands forced him to act. He could not wait for the long expected council to convene and resolve the differences. When two Catholic princes-George, Duke of Saxony, and Joachim I, Elector of Brandenburg-died, each to be succeeded by sons with Protestant convictions, he had to proceed quickly to avoid further religious division and to unite his kingdom militarily. For this purpose he was able to use Landgrave Philip of Hesse. Philip had entered into a bigamous marriage, which was punishable by death according to imperial law, so Charles could count on his support for future religious conversations and agreements.
The first attempted colloquy was scheduled for Speyer in April 1540, but was moved to Hagenau due to illness. When it finally convened, conversations broke down quickly over basic procedural questions. Lutherans wanted the Augsburg Confession as basis for the talks, but the Catholic party argued that it was no longer representative of Luther’s theology, especially in light of the 1537 Smalcald Articles. Due to the gridlock, the emperor declared recess in July and convoked another colloquy for Worms in October. The Worms colloquy eventually opened in November and both sides agreed to accept the Augustana as basis for debate. Only tentative agreement on original sin had been reached between John Eck and Philipp Melanchthon at Worms before the colloquy was suspended again and scheduled to reconvene at Regensburg in April 1541 so Charles V could be present. During the course of Worms, however, surreptitious conversations between the two sides were initiated. With Johannes Gropper and Gerhard Veltwyck on the Catholic side and Martin Bucer and Wolfgang Capito on the other, the resulting conversations led to the so-called Worms Articles, which they would then circulate amongst various nobles and theologians, including Luther and Melanchthon. They were then emended, translated from Latin to German, and placed before the participants at the Regensburg Colloquy (alternately, Diet of Ratisbon).
The result was the infamous Regensburg Book that would serve as the basis for the colloquy in April and May of 1541. The Catholic representatives were Eck, Gropper, and the humanist Julius Pflug, while the Protestants were Bucer, Melanchthon, and Johannes Pistorius. Over those two months, the colloquy led to agreement on numerous articles, including the fall, free will, the cause of sin, and original sin. Eventually compromise was even reached on justification, with both sides agreeing—albeit reluctantly—to a doctrine of double righteousness that balanced a largely Catholic inherited righteousness with a Protestant imputed righteousness. Despite those achievements, the sides could not reconcile their views on the church, the relationship between church and Scripture, or the sacraments, leading to the dissolution of the colloquy. Both Rome and the Protestants ultimately rejected the agreements reached there. Charles V laid aside efforts at religious reconciliation and turned to military concerns, trusting the pope to convoke the promised council and preparing for war against the resisting Schmalkaldic League.