Charles V., relying upon the aid of the Pope and the co-operation of the Catholic princes, issued a proclamation calling upon all loyal subjects to treat them as rebels and outlaws. Maurice of Saxony deserted his co-religionists on promise of succeeding to the Electorship, joined the standard of Charles V., and in conjunction with Ferdinand directed his forces against Saxony. The Elector was defeated and captured at Muhlberg (April 1547). He was condemned to death as a traitor, but he was reprieved and detained as a prisoner in the suite of the Emperor, while his nephew, Maurice of Saxony, succeeded to his dominions. Philip of Hesse, too, was obliged to surrender, and Charles V. found himself everywhere victorious. He insisted on the restoration of the Bishop of Naumburg and of Henry of Brunswick to his kingdom as well as on the resignation of Hermann Prince von Wied, Archbishop of Cologne. He was unwilling, however, to proceed to extremes with the Protestant princes, well knowing that he could not rely on some of his own supporters. Besides, he had become involved in serious difficulties with Pope Paul III., who complained, and not without reason, of the demands made upon him by the Emperor, and of the concessions that the Emperor was willing to make to the Lutherans.
Charles V. summoned a Diet to meet at Augsburg (1547), where he hoped that a permanent understanding might be secured. A document known as the /Augsburg Interim/, prepared by Catholic theologians in conjunction with the Lutheran, John Agricola, was accepted provisionally by both parties. The doctrines were expressed in a very mild form, though not, however, altogether unacceptable to Catholics.
Protestants were permitted to receive communion under both kinds;their married clergy were allowed to retain their wives; and it was understood tacitly that they might keep possession of the ecclesiastical property they had seized. The /Augsburg Interim/, as might have been anticipated, was displeasing to both parties. Maurice of Saxony, unwilling to give it unconditional approval, consulted Melanchthon and others of his school as to how far he might accept its terms. In their reply they distinguished between matters that were essential and those that were only of secondary importance. The latter might be accepted unreservedly in obedience to the orders of the Emperor. In regard to doctrines, they were willing to compromise on the question of justification and good-works, to accept the sacraments, including confirmation and Extreme Unction, the Mass with the addition of some German hymns, and in a certain sense the jurisdiction of the bishops. Such concessions were a distinct departure from Luther's teaching and would have been impossible had he been alive.
The relations between the Pope and the Emperor took a more friendly turn when the General Council was transferred from Bologna to Trent (1551). The Protestant princes, invited to send representatives, declined at first, but in a short time several of them agreed to accept the invitation. Safe conducts were issued for their representatives by the Council in 1551 and again in 1552. Even the Wittenberg theologians were not unfavourably disposed, and Melanchthon was actually on his way to Trent. But suddenly Maurice of Saxony, who had assembled a large army under pretext of reducing Magdeburg, and had strengthened himself by an alliance with several princes as well as by a secret treaty with Henry II. of France, deserted the Emperor and placed himself at the head of the Protestant forces. When all his plans were completed he advanced suddenly through Thuringia, took Augsburg, and was within an inch of capturing the Emperor who then lay ill at Innsbruck (1552). At the same time the French forces occupied Lorraine. Charles, finding himself unable to carry on the struggle, opened negotiations for peace, and in 1552 the Treaty of Passau was concluded. Philip of Hesse was to be set at liberty; a Diet was to be called within six months to settle the religious differences; in the meantime neither the Emperor nor the princes should interfere with ******* of conscience; and all disputes that might arise were to be referred to a commission consisting of an equal number of Protestant and Catholic members.
Owing to the war with France it was not until the year 1555 that the proposed Diet met at Augsburg. The Protestant party, encouraged by their victories, were in no humour for compromise, and as it was evident that there was no longer any hope of healing the religious division in the Empire, it was agreed that peace could be secured only by mutual toleration. In September 1555 the Peace of Augsburg was concluded. According to the terms of this convention full ******* of conscience was conceded in the Empire to Catholics and to all Protestants who accepted the Augsburg Confession. The latter were permitted to retain the ecclesiastical goods which they had already acquired before the Treaty of Passau (1552). For the future each prince was to be free to determine the religion of his subjects, but in case a subject was not content with the religion imposed on him by his sovereign he could claim the right to migrate into a more friendly territory.
A great difficulty arose in regard to the disposal of the ecclesiastical property in case a Catholic bishop or abbot should apostatise. Notwithstanding the protests of the Protestant party, it was decreed that if such an event should occur the seceder could claim his own personal property, but not the property attached to his office. This clause, known as the /Ecclesiasticum Reservatum/, gave rise to many disputes, and was one of the principal causes of the Thirty Years' War.