In a way, Peter by his confession became the
first stone to be built into the structure.
Others following him would become part of it
also. "Now therefore ye are no more strangers
and foreigners, but fellow citizens with the
saints, and of the household of God, And are
built upon the foundation of the apostles and
prophets, Jesus Christ Himself being the chief
corner stone. . . . read Ephesians 2:19-22

But that Peter would become the foundaton is
ridiculous. Jesus is the foundation, Peter and
the others only have a part in it.