His younger brother John succeeded to the throne under the title John III. He was a man of considerable ability, and was by no means satisfied with the new religion. His marriage with Catharine, sister of Sigismund, King of Poland, herself a devoted Catholic, who stipulated for liberty to practice her religion, helped to make him more favourable to a Catholic revival. He set himself to study the Scriptures and writings of the Holy Fathers under the guidance of Catharine's chaplains, and convinced himself that he should return to the Catholic Church and endeavour to rescue his country from the condition of heresy into which it had fallen. He allowed the monks and nuns who were still in Sweden to form communities again, and endeavoured to win over the clergy by a series of ordinances couched in a Catholic tone which he issued for their guidance. In 1571 he induced the Archbishop of Upsala to publish a number of regulations known as the /Agenda/, which both in ritual and doctrine indicated a return to Rome, and he employed some Jesuit missionaries to explain the misrepresentations of Catholic doctrine indulged in by the Lutheran and Calvinist leaders. His greatest difficulty in bringing about a reunion was the presence of Lutheran bishops, but fortunately for him many of them were old men whose places were soon vacant by death, to whose Sees he appointed those upon whom he could rely for support. When he thought the time was ripe he summoned a National Synod in 1574, where he delivered an address deploring the sad condition to which religious dissensions had reduced the country. He pointed out that such a state of affairs had been brought about by the Reformation and could be remedied only by a return to the Church. The address received from the clergy a much more favourable reception than he had anticipated. As the Archbishopric of Upsala was vacant, he secured the election of an archbishop, who have his adhesion to seventeen articles of faith wholly satisfactory to Catholics, and who allowed himself to be consecrated according to the Catholic ritual. He promised also to use his influence to secure the adhesion of the other bishops. In 1576 the king issued a new liturgy, /The Red Book of Sweden/, which was adopted by the Diet in 1577, and accepted by a large body of the clergy. Its principal was the king's brother, Karl, Duke of Suthermanland, who for political reasons had constituted himself head of the Lutheran party, and who refused to agree with the Roman tendencies of the king on the ground that they were opposed to the last wishes of Gustavus and to the laws of Sweden. A disputation was arranged to take place at Upsala, where the Belgian Jesuit, Laurence Nicolai, vindicated triumphantly against his Lutheran opponents the Catholic teaching on the Church and the Mass. Copies of the celebrated catechism of the Blessed Peter Canisius were circulated throughout Sweden, and made an excellent impression on the people.