Francis’ opening was part of his mercy-before-doctrine drive, which welcomes outcasts to the church rather than use dogma to keep people out. But for conservative Catholics, it was another reason to feel aggrieved, after Francis appeared to suggest in 2015 that Lutherans could receive Catholic Communion, and, when asked about gay people in 2013, said, "Who am I to judge?"
It was enough to start talk of a schism in the church. In February, anonymous conservatives in Rome responded by putting up posters attacking the pope, while rumors spread of prelates plotting to make him resign.
Mueller’s senior role as the the head of the Vatican’s Congregation for the Doctrine of the Faith gave his criticism extra weight. But his nationality also provides a key to understanding the roots of the row — one that has been raging for years and started not in the Vatican, but in Germany, long before Francis was pope.