The fact that it is Peter who is called the “rock” is not due to any achievement on his part or to anything exceptional in his character; it is simply a nomen officii, a title that designates, not a service rendered, but a ministry conferred, a divine selection and commission to which no one is entitled solely by virtue of his own character -least of all Simon, who, if we are to judge by his natural character, was anything but a rock.
By nature he is that Peter who sinks into the waves when his faith fails; it is by the Lord and through the grace of the Lord that he is the rock on which the Church stands. We have grown accustomed to make a clear distinction between Peter the rock and Peter the denier of Christ – the denier of Christ: that is Peter as he was before Easter; the rock: that is Peter as he was after Pentecost, the Peter of whom we have constructed a singularly idealistic image. But, in reality, he was at both times both of these.
The pre-Easter Peter is already the Peter who, when many of the disciples were abandoning Jesus, spoke for those who remained faithful; who walked on the water to meet his Lord; who uttered the inexpressibly beautiful words: “Lord to whom can we go? You have the words of the eternal life. We have come to believe and know that you are the Holy One of God” (Jn 6:68-69).
The post-Pentecost Peter, on the other hand, is that same Peter who, for fear of the Jews, belied his Christian freedom (Gal 2:11-14); he is at once a rock and a stumbling block. And has it not been thus throughout the history of the Church that the Pope, the successor of Peter, has been at once Petra and Skandalon -both the rock of God and a stumbling block? In fact, the faithful will always have to reckon with this paradox of the divine dispensation that shames their pride again and again -this tension between the Rock and Satan, in which the most extreme opposites are so strangely interwoven.
From: Das neue Volk Gottes, pp.80ff