The pope and the cardinal clashed over the various reform measures, including the hot-button issue of annulments. Cardinal Müller was also allegedly lax in responding to sex abuse claims brought before the Vatican.
Pope Francis shook-up the Vatican's administration Saturday by replacing the church's top theologian with whom he often clashed.
Pope Francis chose not to renew the five-year term of 69-year-old German Cardinal Gerhard Ludwig Müller, who headed the Congregation of the Doctrine of Faith, which is responsible for defending Catholic doctrine.
Instead, the pope has turned to the department's deputy, Archbishop Luis Francisco Ladaria Ferrer. Like Francis, Ladaria is a Jesuit with a more liberal view of theology.
The pope has upset conservatives by suggesting that a ban on remarried divorcees taking Holy Communion could, in carefully scrutinized circumstances, be overcome without the traditional annulment. Müller, one of the pontiff's chief critics, said such an approach undermined Catholic dogma on the permanent nature of marriage.
In 2015 Müller was among 13 cardinals who signed a secret letter to the pontiff complaining that a meeting of bishops discussing family issues was stacked in favor of liberals. The letter was leaked to the media, embarrassing the signatories.
Pope, cardinal spar
German theologian Wolfgang Beinert said Müller and Francis never got along.
"They are chemistry-wise two different people who are incompatible by nature," he said. Beinert described the pope's decision to cut ties with Müller as a "punishment."
A priest, who works at the Vatican, and knows both Müller and his replacement, Ladaria, said the latter will get along with Pope Francis much better.
"They speak the same language and Ladaria is someone who is meek. He does not agitate the pope and does not threaten him," said the priest, who didn't want to be identified.
Since becoming pope in 2013, Francis has given hope to more progressive voices inside the church who want him to push ahead with his vision for a more tolerant church that focuses more on mercy than on a strict enforcement of rigid rules, which they see as outdated.
Müller's ouster was the second major shakeup at the Vatican this week. On Thursday, Francis granted Vatican hardliner Cardinal George Pell a leave of absence to return to his native Australia to face trial on sexual assault charges.
The absence of Müller and Pell, the two most powerful cardinals in the Vatican, after the Vatican secretary of state, Cardinal Pietro Parolin, will likely create a power vacuum for the conservative wing in the Holy See hierarchy.
Müller was also allegedly lax in addressing sexual abuse cases that have come before the Vatican. During his tenure victims from Latin America, Europe and beyond came forward to press their cases.
Last year the pope confirmed there was a 2,000-case backlog, and he set about naming new officials in the congregation's discipline section to process the overload.
bik/jm (Reuters, AP, dpa)