@Pontifex (underscore the language he is tweeting in):
This handle is based on the title "Pontifex Maximus," Latin for "Greatest Bridgemaker." The Pope has this title because it was given to the Bishops of Rome centuries ago by a Roman Emperor, who didn't think Christian Emperors should hold that previously pagan office any longer.
Here's the story.
The office of Pontifex Maximus was instituted by the Roman King Numa Pompilius (reigned 715 - 673 BCE), who first bestowed it upon his son Numa Marcius (Livy, Ab Urbe Condita 1.20).
The concept of "Bridgemaker" means that this chief priest is the bridgemaker between the human and divine realms.
The office was then continuously bestowed or achieved through election into the Imperial period.
On February 27, 380 CE, Emperor Gratian, who, as a Christian, did not want to further hold the office and title of Pontifex Maximus, issued an edict in which he declared that the Bishop of Rome, Damasus (and his successors) was the Pontifex.
The Emperor of Rome was understood to have the authority to bestow this office and title upon whomever he wished. As a result, the office did not vanish, it merely changed religions.
Bishops of Rome have not always listed Pontifex Maximus among their various other titles such as, Patriarch of the West, etc.
But when Benedict XVI started the Papal Twitter account, he claimed the title once again, choosing the handle:
@Pontifex (underscore the language of the tweet)
And as I said above, Pope Francis has taken over these accounts and continued to use the title accordingly.
As a result, it is reasonable to assert that a man today in 2016 is still legitimately holding an office and title first instituted about 2700 years ago.
And this would make the office and title of Pontifex Maximus easily the longest held institution in human history.
You'll travel back to ancient Rome on a harrowing mission to save the modern world. It's the adventure of four lifetimes.