There is a letter attributed to Clement, alleged bishop of Rome (i.e., pope) toward the end of the first century, although no contemporary documents support Clement's reign or even his existence. Nevertheless, the Church still claims him as the third pope after Peter, whose existence is equally dubious. Clement's letter holds a definition of heresy that actually belongs to a much later century, after consolidation of the Church's temporal power. It says that "authority of reign" is designated by God to the priests, deacons and bishops on Earth, and whoever refuses to "bow the neck" to them is guilty of insubordination against God and must receive the death penalty.
On the basis of this probably spurious letter, the Church declared war on all its critics: Gnostics, pagans, unbelievers and other dissidents. In Against All Heresies (3.3.2), Church father Irenaeus wrote: "For it is a matter of necessity that every Church should agree with this [Roman] Church, on account of its preeminent authority...." Heresy was defined in Rome as "insubordination to clerical authority," meaning that anyone who disagreed with anything a priest said was by definition a heretic. Of course the Church fathers themselves argued bitterly on all points of theology and murdered one another with considerable enthusiasm. One scholar points out that whenever the clergy met in large numbers, as at a council, they chose a town near a large body of water for disposal of the bodies. It was said that Lake Constance received some 500 corpses during the council that met there and the Rhine River received many more.
Christians persecuted other Christians with as much fervor as they spent on killing pagans. In 385, the pious Bishop Priscillus of Spain and six of his followers were first tortured and then beheaded for holding certain doctrines associated with the Manichaean heresy. The fifth-century pope Leo the Great endorsed the death penalty for all "erroneous beliefs." Pope Urban II declared in the 10th century that all heretics must be tortured and killed; and Pope Innocent III (who was certainly not innocent) stated that everyone must obey the pope even if what he commands is evil. He further stated that anyone holding a personal view of God that conflicts in any way with Church dogma "must be burned without pity."
It is interesting, though, to note that of the 80 or so heresies during the first six Christian centuries, not one makes any reference to the claim to authority of the Roman pontiff, because no one had yet heard of it. Only the Catholic Church postulates an unbroken succession of allegedly infallible popes dating from St. Peter. There is no record of it anywhere else.
For more, see Man Made God.