A Prayer Rule by St. Theophan the Recluse

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Orthodox Information Center

Did Cardinal Ratzinger think that Vatican I is non-negotiable?

It would seem so, with respect to its teachings on the office of the pope. From the CDF's Doctrinal Commentary on Ad Tuendam Fidem:

With respect to the truths of the second paragraph, with reference to those connected with revelation by a logical necessity, one can consider, for example, the development in the understanding of the doctrine connected with the definition of papal infallibility, prior to the dogmatic definition of the First Vatican Council. The primacy of the Successor of Peter was always believed as a revealed fact, although until Vatican I the discussion remained open as to whether the conceptual elaboration of what is understood by the terms 'jurisdiction' and 'infallibility' was to be considered an intrinsic part of revelation or only a logical consequence. On the other hand, although its character as a divinely revealed truth was defined in the First Vatican Council, the doctrine on the infallibility and primacy of jurisdiction of the Roman Pontiff was already recognized as definitive in the period before the council. History clearly shows, therefore, that what was accepted into the consciousness of the Church was considered a true doctrine from the beginning, and was subsequently held to be definitive; however, only in the final stage - the definition of Vatican I - was it also accepted as a divinely revealed truth.

I don't see why he would have changed his mind once become pope.