German Cardinal Walter Brandmüller, widely seen as a key opponent of Pope Francis, has penned a rare essay openly criticizing the upcoming Synod of Bishops on the Amazon, saying the official preparatory document breaks with Catholic teaching.

According to Brandmüller’s essay, published on June 27, on the Settimo Cielo blog of Italian journalist Sandro Magister, the Synod’s recently published preparatory document “burdens the Synod of Bishops, and finally the Pope, with a grave breach with the depositumfidei, which in its consequence means the self-destruction of the Church or the change of the Corpus Christi mysticum into a secular NGO with an ecological-social-psychological mandate.”

Published on June 17, the document, known as the instrumentum laboris, will set the ground work for the Synod of Bishops on the Amazon, that will take place in Rome on Oct. 6-27. Among other things, it opened a cautious door to the ordination of married “elders” as a possible solution to the region’s priest shortage. In his essay, Brandmüller insisted that “the Instrumentum Laboris contradicts the binding teaching of the Church in decisive points and thus has to be qualified as heretical.”

“Inasmuch as even the fact of Divine Revelation is here being questioned, or misunderstood, one also now has to speak, additionally, of apostasy,” he said, voicing grievances over the document’s take on liturgy, spirituality and priestly celibacy.