From Wikipedia:
I suppose that it would be rather difficult for him to succeed but if he did, the consequences might be rather big for whole of Europe. (Thirty Years' War without Sweden?)
What if John III had actually converted into Catholicism and started a counter-reformation?Attempts of Catholic reunification
Under Eric XIV the Reformation in Sweden proceeded on the same lines as during the reign of his father, retaining all the old Catholic customs not considered contrary to Scripture. After 1544, when the Council of Trent had formally declared the Bible and tradition to be equally authoritative sources of all Christian doctrine, the contrast between the old and the new teaching became more obvious; and in many countries a middle party arose which aimed at a compromise by going back to the Church of the Fathers. King John III of Sweden, the most learned of the Vasas, and somewhat of a theological expert, was largely influenced by these middle views. As soon as he had mounted the throne he took measures to bring the Church of Sweden back to "the primitive Apostolic Church and the Swedish Catholic faith"; and, in 1574, persuaded a synod, assembled at Stockholm, to adopt certain articles framed by himself. In February 1575 a new Church ordinance, approximating still more closely to the patristic Church, was presented to another synod and accepted, but very unwillingly. In 1576 a new liturgy was issued on the model of the Roman missal, but with considerable modifications.
Despite the opposition of Duke Charles and the ultra-Protestants, these measures were adopted by the Riksdag of the Estates in 1577. They greatly encouraged the Catholic party in Europe, and John III was ultimately persuaded to send an embassy to Rome to open negotiations for the reunion of the Swedish Church with the Holy See. But though the Jesuit Antonio Possevino was sent to Stockholm to complete John’s conversion, John would only consent to embrace Catholicism under certain conditions which were never fulfilled, and the only result of all these subterraneous negotiations was to incense the Protestants still more against the new liturgy, the use of which by every congregation in the realm without exception was, nevertheless, decreed by the Riksdag of 1582.
At this period Duke Charles and his Protestant friends were clearly outnumbered by the promoters of the via media. Nevertheless, immediately after King John's death, the Uppsala Synod, summoned by Duke Charles, rejected the new liturgy and drew up an anti-Catholic confession of faith, March 5, 1593. Holy Scripture and the three primitive creeds were declared to be the true foundations of Christian faith, and the Augsburg confession was adopted.
I suppose that it would be rather difficult for him to succeed but if he did, the consequences might be rather big for whole of Europe. (Thirty Years' War without Sweden?)