Was he married like Peter was?
"The earliest record of the use of this title was in regard to the by then deceased Patriarch of Alexandria, Pope Heraclas of Alexandria (232248)." [Eusebius, Historia Ecclesiastica Book VII, chapter 7.7]
"The title was from the early 3rd century a general term used to refer to all bishops. From the 6th century, the title began to be used particularly of the Bishop of Rome, and in the late 11th century Pope Gregory VII issued a declaration that has been widely interpreted as stating this by then established Western convention. By the same 6th century, this was also the normal practice of the imperial chancery of Constantinople. [ "Pope", Oxford Dictionary of the Christian Church, Oxford University Press, 2005, ISBN 978-0-19-280290-3]
The Catholic history of the title pope is fallacy.
I think they mean the first six decades of the Church. The first six centuries happened five hundred years after he died.