Et ego dico tibi quia tu es Petrus et super hanc petram aedificabo ecclesiam meam et portae inferi non praevalebunt adversum eam. (Matt 16:18, Vulgate)
And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it. (Matt 16:18 DR)
While some Catholic apologists believe that the interpretation of the “rock” in Matt 16:18 being anything/anyone else but the person of Peter is a “Protestant” interpretation, the truth is that such pre-existed the Protestant Reformation. Indeed, even when the papacy was firmly established, many Catholic canonists and other theologians did not believe that Peter was the primary referent of “this rock” (Latin: hanc petram). As Brian Tierney noted the varying interpretations which were acceptable in this era:
Stephanus Tornacensis and Simon de Bisignano thought that the Rock on which the Church was founded must be either Christ himself or the unshakable faith that Peter had displayed. Joannes Faventinus added to these interpretations the suggestion that the Rock might be Peter himself, and the Summa Parisiensis explained that the Church was founded principally on Christ, secondarily on Peter. This point was developed by Huguccio in a lengthy gloss which summarized the various earlier opinions; the Rock might be either Christ or Peter or Peter’s faith, and although the Church was founded on Christ principaliter et tanquam auctorem, it could be regarded as founded on Peter secundario et quasi ministrum. In the next generation the Glossa Palatina and the gloss Ecce Vicit Leo identified the Rock with Christ himself, while Joannes Teutonicus, in his Glossa Ordinaria, stated emphatically his opinion that it was best understood as the article of faith that Peter had enunciated.
Et super hanc petram. Per hanc dictionem non credo Dominum aliud demonstrasse quam hae verba quae Petrus respondit Domino cum dixit, Tue s Christus, filius Dei vivi, quia super illo articulo fidei fundata est ecclesia. (Glossa Ordinaria ad Dist. 19 c. 7)
The various explanations of St. Matthew’s text other than that which identifies the Rock with Peter himself are sometimes referred to as ‘Protestant’ interpretations; but the twelfth-century canonist saw no incongruity in preferring one of the alternative expositions while at the same time deriving from the text a most exalted conception of the papal dignity and powers. Even if Christ himself was the true Rock, it could still be held that Peter alone was named after the Rock and so was singled out for a position of special eminence among the Apostles; and even though the Church was held to be founded ultimately on Christ, still Peter could be regarded as its founder in a secondary sense, quasi minister, as the chosen instrument of the heavenly Founder. (Brian Tierney, Foundations of the Conciliar Theory: The Contributions of the Medieval Canonists from Gratian to the Great Schism [Cambridge: Cambridge University Press, 1955], 26-28; cf. Tierney, Origins of Papal Infallibility 1150-1350: A Study On the Concepts of Infallibility, Sovereignty and Tradition in the Middle Ages [Leiden: Brill, 1997] and John F. Bigane, III, Faith, Christ or Peter: Matthew 16:18 in Sixteenth Century Roman Catholic Exegesis [Washington: University of America Press, 1981])
Erasmus, another “heavy hitter” in Catholic theology, held to a similar view of Matt 16:18. See: