It's highly questionable to say that the words we read now as the prophecy of Our Lady of La Salette are truly the prophecy of La Salette. The story is rather more complex.
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When Melanie received the apparition, she wrote down her secret and sent it to Pope Pius IX. This text is the true text of the secret and has never been published, and remains secret to this day.
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Later in her life, about 30 years after the apparition, Melanie published a text that she claimed was what Our Lady had told her as a child. This is the text people are quoting from when they quote the "prophecy" of La Salette. The problem with this text is that the Holy Office immediately placed it on the Index of Forbidden Books and forbade anyone from speaking of this text. This law was repeated a few years later when people ignored it and continued to discuss this text, despite its listing on the Index of Forbidden Books. The Holy Office further clamped down by making a law that anyone who discussed this text would be refused the sacraments, if lay people, and suspended a divinis if they were priests (forbidden from any function of their ministry).
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So obviously any argument based on a quote from the Indexed text is highly problematic.