"Along with Holy Scripture, divinely inspired, stands the authority of the Church. He follows the words of Saint Augustine: 'I would not believe in the Gospel, if first I did not believe in the Church.' In fact, our Doctor places in an elevated position, the supreme authority of the Successor of Peter. According to his words, 'although the Pope cannot establish against natural and divine law (because his power is inferior to both), nevertheless, being the Successor of Peter, the Prince of the Apostles, he has the same authority that Peter had.'
Thanks to Rorate Caeli, where one may read more on this letter under the date of 20 December 2008.