They said that some bishops and cardinals have been "silent" in response to this "drift" in the Church, and asked those attending this week's conference in Rome if they would "also be silent."
Burke and Brandmüller are the two living members of a group of four so-called "dubia" cardinals who submitted formal requests for clarification to Pope Francis regarding the interpretation of his apostolic exhortation Amoris Laetitia, published after the Synod on the Family.
In the letter, Brandmüller and Burke note that they have not yet had a response to the dubia, and suggest that the need for clarification is "part of a more general crisis of the faith."
"Therefore, we encourage [bishops] to raise your voice to safeguard and proclaim the integrity of the doctrine of the Church," they wrote. "A decisive act now is urgent and necessary."
The letter was released just days before the world-wide summit in Rome to address the sexual abuse crisis, and ahead of the publication of a widely trailed book entitled "In the Closet in the Vatican." Authored by a French journalist, the book purports to expose a cultutre of homsexuality, hypocrisy, and secrecy in the upper ranks of the curia.