Saint Peter, whom Jesus called to "feed my sheep," to shepherd the fledgling Christian community, to lead his infant church, was a flawed human being. (Even the nickname apparently bestowed on him by Jesus—Cephas, or Peter, meaning "Rocky"—affirms the angularity of Peter's character.) From the beginning, Peter was acutely aware of his own sinfulness and own weakness. Near the opening of the Gospel of Luke, when Jesus first meets Peter and performs a miracle before him, Peter cowers in shame, and says, "Depart from me, Lord, for I am a sinful man."
This is not false humility. They are the words of someone who, in the face of the divine mystery, manifests a profound understanding of his own sinfulness and personal limitations. It is also a natural response to the transcendent. Such an awareness is central to the development of the Christian life: it is the sign of a humility that marks the beginning of a true relationship with God. This is the reason that early in his Spiritual Exercises, St. Ignatius Loyola, the founder of the Jesuit order, asks the person making the retreat to pray for "an interior knowledge of my sins...and the disorder of my actions."
Anyone who reads even a few gospel stories will encounter what you could call the "historical Peter." He is headstrong, doubtful, confused, impulsive: human. The reader will also discover that Jesus loves Peter and loves him with abandon. Jesus is constantly offering him forgiveness, even for his craven behavior at the crucifixion, and is consistently placing his trust in him. (Scripture scholars note that when Jesus asks Peter "Do you love me?" three times by the seashore, it is meant to counterbalance Peter's triple denial of Jesus before the crucifixion.)
Peter is among the greatest of the saints precisely because of his humanity, his shortcomings, his doubt and, moreover, his deeply felt understanding of all these things. Only someone like Peter, who understood his own sinfulness and the redeeming love of Christ, would be able to lead the infant church and lead others to Jesus.
Only someone as weak as Peter would be able to do what he did.
from My Life with the Saints, by James Martin, SJ