After the break between the Catholics and the Protestant reformers, moderate Catholic rulers such as the Holy Roman Emperor Charles V tried to reconcile the two sides. At his instigation, Protestant theologians developed a statement of principles called the Confession of Augsburg that was acceptable to some Catholics. Even Charles V conceded in 1530 (and here I'm paraphrasing), "if that's how you feel, we're not so far apart, and we can agree to disagree," allowing for a temporary and limited reconciliation between the two sides. This document was signed by a young priest named John Calvin.
About 30 years later, a then-moderate ruler, Catherine de Medici of France, tried a similar reconciliation between the Catholics and Calvinists. But with the latter group, it was a case of their being "more royalist than the king, more Catholic than the pope". When they were asked at the Colloquy at Poissy whether they would subscribe to the Apology of Augsburg (which their own leader, John Calvin, had signed) as a first step toward reconciliation, their answer was a pointed "no." From then on, the breach between them and the Catholics got worse, leading to the St. Bartholomew's massacre of 1574.
All this pointed out that Calvinism was too extreme even for Catholics that were willing, or least preferred, to come to terms with Lutheranism. The latter religion preached that salvation came by faith alone, and opposed the Catholic "sale of indulgences," the Catholic idea that salvation could be "bought" by donations to the church. But Calvinist preached that mankind was "totally depraved," and therefore damned, except by the arbitrary exercise of God's forgiveness, a doctrine too extreme for most theologians.