"Upon this rock I will build my church."
OPINION: Not from these words alone does the Catholic Church trace its universal authority.
But Jesus's bestowal of the name "Peter" or Petros (the Greek word for "rock") upon Simon Bar-jonah, the first of his disciples to recognise him as the Christ, is by far the most familiar justification for what Catholic scholars call the "Petrine Succession".
The full quotation from the Book of Matthew bears repeating. When Jesus asks his followers, "Who do you say that I am?", only Simon answers "correctly".
"You are the Christ," says the fisherman, "Son of the living God." Blessing him, Jesus goes on to offer his famous benediction: "And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it. And I will give unto thee the keys of the kingdom of heaven."
As anyone who has ever visited the Vatican will tell you, the papal symbol of the crossed keys are everywhere.
That's because, according to Catholic tradition, the Bishopric of Rome (which, by virtue of Rome's supremacy, confers leadership of the entire Church) has been passed, through God's divine guidance, directly and in unbroken succession from Jesus's right-hand-man to the present Bishop of Rome, Joseph Ratzinger - Pope Benedict XVI.
The very same pontiff who, on Tuesday morning, stunned the entire, one-billion-strong, Catholic Church by announcing his abdication.
Can a pope really do that?
Well, it has been done before. Nearly 600 years ago Pope Gregory XII relinquished the papal throne - but only because the two other princes of the church who were at that time also claiming to be pope had bigger armies at their backs.
Since 1415, however, the pontificate has only been vacated when God called the incumbent home.
Given the proposition that it is God, himself, who guides the deliberations of the College of Cardinals (one of whose number will become the next pope) this seems only right and proper.
After all, when Simon identified him as the Christ, Jesus said: "Blessed art thou, Simon Bar-jonah: for flesh and blood hath not revealed it unto thee, but my Father who is in heaven."
In other words the authority of the "rock", upon whom Jesus would build his church, was not the product of Simon-Peter's own will - born of flesh and blood - but of the will of God. Peter's authority is God's authority. If it were otherwise the Petrine Succession would make no sense.
So, is it really possible to step away from God's will? Is the pontificate an office from which one can calmly announce one's resignation? Is being the pope just like being a chief executive? And the papacy nothing more than a job?
Pope Benedict's predecessor certainly didn't think so.
As Pope John-Paul II's body was slowly and agonisingly broken on the cross of Parkinson's disease, the doughty Polish pontiff wore his pain as a blessing, saying it brought him closer in spirit to the suffering of Christ.
How many times must he have recalled the words of Jesus in the Garden of Gethsemane: "Father, if thou be willing, remove this cup from me: nevertheless not my will, but thine, be done."
Lyndsay Freer, speaking on behalf of the Auckland Catholic Diocese, told TV3's Firstline: "I think that Pope Benedict has clearly become frailer, and it is a huge burden of office that he carries . . . I think he feels that out of respect for that office and the ministry that's been entrusted to him, it's time for somebody younger, stronger to take over."
But that would make the Petrine Succession a purely human construction, and the pope nothing more than a holy chief executive.
According to Catholic legend, as Peter fled from certain crucifixion in Rome he encountered Jesus walking in the opposite direction.
"Quo vadis?" asked Peter. "Whither goest thou?"
"To Rome, to be crucified," Jesus replied.
Peter turned around.
- The Dominion Post