Luther also showed up just as the HRE's centralization was beginning to show major major cracks. Luther wasn't the first "heretic" to challenge the catholic church, but he was the first one with major political support (a duke or a prince, I can't recall which).
There's also the Moravians and Lolards. Everyone knew Catholicism needed to change but it wasn't until the Reformation the pressure existed for the Counter-reformation to overcome the obstacles within the Catholic church.
