“If your church was built on Simon Peter you have a rough foundation.”

This is kind of abstract but see this through.
In a debate between Karl Keating and Dr. Peter Ruckman the latter stated the following:

“If your church was built on Simon Peter you have a rough foundation.”

The thing is that God can use anyone he wants to do his will without defeating their own free will, but he always chooses the least so they can be seen as magnifying his glory. 

Consider the following examples:

Gideon’s remaining army
David
St. Mary Alacoque
St. Bernadette

Protestants tend to believe that Jesus wouldn’t use such a broken vessel like Peter who can do such good then have to be corrected sternly by Paul. It would be easy for God to take hold of Alexander the Great or a great Paraoh to do his will, but how much more amazing is it for him to use a sinner like you and I to change the whole world?

There is a beauty in God’s plan that we cant see often because our sin has darkened our vision.  I think GK Chesterton out of all people takes on and answers the Why Peter question the best in his book Heretics:

“Now this is, I say deliberately, the only defect in the greatness of Mr. Shaw, the only answer to his claim to be a great man, that he is not easily pleased. He is an almost solitary exception to the general and essential maxim, that little things please great minds. And from this absence of that most uproarious of all things, humility, comes incidentally the peculiar insistence on the Superman. After belaboring a great many people for a great many years for being unprogressive, Mr. Shaw has discovered, with characteristic sense, that it is very doubtful whether any existing human being with two legs can be progressive at all. Having come to doubt whether humanity can be combined with progress, most people, easily pleased, would have elected to abandon progress and remain with humanity. Mr. Shaw, not being easily pleased, decides to throw over humanity with all its limitations and go in for progress for its own sake. If man, as we know him, is incapable of the philosophy of progress, Mr. Shaw asks, not for a new kind of philosophy, but for a new kind of man. It is rather as if a nurse had tried a rather bitter food for some years on a baby, and on discovering that it was not suitable, should not throw away the food and ask for a new food, but throw the baby out of window, and ask for a new baby. Mr. Shaw cannot understand that the thing which is valuable and lovable in our eyes is man—the old beer-drinking, creed-making, fighting, failing, sensual, respectable man. And the things that have been founded on this creature immortally remain; the things that have been founded on the fancy of the Superman have died with the dying civilizations which alone have given them birth. When Christ at a symbolic moment was establishing His great society, He chose for its corner-stone neither the brilliant Paul nor the mystic John, but a shuffler, a snob a coward—in a word, a man. And upon this rock He has built His Church, and the gates of Hell have not prevailed against it. All the empires and the kingdoms have failed, because of this inherent and continual weakness, that they were founded by strong men and upon strong men. But this one thing, the historic Christian Church, was founded on a weak man, and for that reason it is indestructible. For no chain is stronger than its weakest link.”