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In separate Aug. 28 letters to the College of Cardinals, the dubia authors warned brother bishops to guard against attempts to overthrow Church teaching.
Cardinal Brandmüller noted that the Instrumentum laboris — the synod's working document — is rife with heterodoxy: "Some points ... seem not only in dissonance with respect to the authentic teaching of the Church, but even contrary to it," he said.
"The nebulous formulations of the Instrumentum, as well as the proposed creation of new ecclesial ministries for women and, especially, the proposed priestly ordination of the so-called viri probati arouse strong suspicion that even priestly celibacy will be called into question," the cardinal warned.
Brandmüller urged fellow cardinals to prepare to counter "any heretical statements or decisions of the synod," adding: "We must face serious challenges to the integrity of the Deposit of the Faith, the sacramental and hierarchical structure of the Church and its Apostolic Tradition."
Underscoring the severity of the looming threat, the cardinal observed: "With all this has been created a situation never before seen in the Church's history, not even during the Arian crisis of the fourth and fifth century."
Cardinal Raymond Burke reinforced these sentiments in his own letter, saying he "shares completely the deep concerns of Cardinal Brandmüller on the upcoming Synod on the Amazon, based upon its Instrumentum laboris."
Burke noted that the working document is "marked by language which is not clear in its meaning, especially in what concerns the Depositum fidei."
The Instrumentum laboris, he wrote, "contradicts the constant teaching of the Church on the relationship between the created world and God, the uncreated Creator, and man, created in the image and likeness of God to cooperate with him as guardian of the created world."
In the document, he continued, "[T]he truth that God has revealed Himself fully and perfectly through the mystery of the Incarnation of the Redeemer, the Son of God, is obscured, if not denied."
"Cardinal Brandmüller indicated in his letter the serious difficulties regarding the ordained ministry and perfect continence of the clergy," said Burke. "These proposals, as the cardinal indicates, attack the 'hierarchical-sacramental structure' and 'the Apostolic Tradition of the Church.'"
The "disturbing propositions of the Instrumentum laboris portend an apostasy from the Catholic faith," he added.
The cardinals voiced alarm over synod leaders, noting their proclivities toward heterodoxy and dissent.
According to Brandmüller: "The sole fact that Cardinal (Cláudio) Hummes is the president of the synod and thus will exercise a grave influence in a negative sense, suffices to have a well founded and realistic concern, as much as in the case of bishops (Erwin) Kräutler, (Franz-Josef) Overbeck, etc."
Hummes, from Brazil, was appointed head of the synod by Pope Francis in May; he serves as president of the Pan-Amazonian Church Network (REPAM), a leading promoter of liberation theology.
Austro-Brazilian Bp. Erwin Kräutler has long advocated for married priests. German Bp. Franz-Josef Overbeck has called for the Church to relax its teaching on homosexuality; in May, he declared the synod will mark a point of no return for the Church, after which "nothing will be the same as it was."
Concerns over the Instrumentum laboris are being compounded by another preparatory document, "Towards the Pan-Amazonian Synod: Challenges and Contributions from Latin America and the Caribbean."
The document stems from an April meeting in Bogotá, Columbia, co-organized by REPAM and Amerindia, another leading promoter of liberation theology.
Rife with heretical statements, it scorns the salvific mission of the Catholic Church, asserting that there is no one, true faith — that all religions are capable of bringing salvation to their followers. It also reduces the Eucharist to a "symbolic" expression of "communal" experience.
The Bogota document praises pagan indigenous traditions, calling for understanding and recognition of "the virtues, knowledge and cosmovisions existing among the ancestral ethnic groups, which still retain the ability to read and conceive nature as the true mother."
Calling for an end to the "patriarchal perspective" of the Church, the document advocates for a "feminist and ecological theology" complete with women priests.
In its closing prayer, it addresses God as "Father and Mother of life" and the "Creator-Creatora."