Why does the RCC so passionately protest and reject this practice?
The RC Denomination rejects the Rule of Scripture in norming not because it rejects Scripture or has an alternative rule that is MORE inerrant, MORE the inscripturated words of God, MORE reliable, MORE objectively knowable, MORE unalterable, MORE ecumenically embraced as authoriative, MORE above and beyond and outside all disputing parties. Rather the rejection is because the protestor rejects accountability (and thus norming and any norm in such) in the sole, singular, exclusive, particular, unique, individual case of it itself alone, uniquely, individually.
From The Handbook of the Catholic Faith (page 151), "When the Catholic is asked for the substantiation for his belief, the correct answer is: From the teaching authority. This authority consists of the bishops of The Catholic Church in connection with the Pope in Rome. The faithful are thus freed from the typically Protestant question of 'is it true' and instead rests in quiet confidence that whatever the Catholic Church teaches is the teaching of Jesus Himself since Jesus said, 'whoever hears you hears me'." The Catholic Church itself says in the Catechism of itself (#87): Mindful of Christ's words to his apostles: “He who hears you, hears me”, the faithful receive with docility the teachings and directives that their pastors give them in different forms."
Since it itself declares that it itself exclusively, individually is unaccountable and that whatever it itself uniquely, individually, currently says is just to be swallowed whole because it itself alone says it and it itself alone tells all to do that in total submission to it itself as unto God Himself, then the entire issue of norming (and the embraced norma normans in such) becomes entirely irrelevant (for itself uniquely). The issue has been changed from truth to the enormous, unmitigated POWER that it itself claims for it itself uniquely that calls on all to just obey, submit, swallow.
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