Saint Peter 

Peter is said to have been the 1st leader of Christianity after the assumed death of Jesus Christ, at least according to the Catholic Church, as I understand it.

Other religious beliefs choose James the Just or Mary Magdalene as having become the Church Leader after the death of Jesus Christ.

Perhaps one of these ideas is correct.  Perhaps none of them are right.

Somewhere along the way the Catholic Church determined that Peter was a Saint and Peter is now usually known as Saint Peter instead of just plain old Peter.

Peter is thought to have been the leader of the Christian movement in Rome from about 42 AD until about 67 AD.  Peter probably was not the overall leader of the new cult of Christianity at the time.  The real leader may have been James the Just of Mary Magdalene. (See "The Second Messiah" p 3 for more information.