In 1568, he was the real leader of the rebellion against Eric XIV. However, he took no part in the designs of his brother John III against the unhappy king after his deposition. Charles's relations with John were always more or less strained. He was at least suspected of being implicated in the Mornay Plot to depose John III in 1574, and was one of the alternative regents suggested by the conspirators of the 1576 Plot. He had no sympathy with John's High-Church tendencies on the one hand, and he sturdily resisted all the king's endeavours to restrict his authority as Duke of Södermanland on the other. The nobility and the majority of the Riksdag of the Estates supported John. However, in his endeavours to unify the realm, and Charles had consequently (1587) to resign his pretensions to autonomy within his duchy. But, steadfast Calvinist as he was, on the religious question he was immovable. The matter came to a crisis on the death of John III in 1592. The heir to the throne was John's eldest son, Sigismund III Vasa, already king of Poland and a devoted Catholic. The fear that Sigismund might re-catholicize the land alarmed the Protestant majority in Sweden—particularly the commoners and lower nobility, and Charles came forward as their champion, and also as the defender of the Vasa dynasty against foreign interference.