Where did the Catholic Church
get the title of POPE or Supreme Pontiff?

In ancient Rome, the Pontifex Maximus was the high priest of the collegium of the Pontifices, the most august position in Roman religion, open only to a patrician (a privileged class that ruled Rome), until 254 B.C., when a plebeian (average Roman citizen) first occupied this post. A distinctly religious office under the early Roman Republic, it gradually became politicized until, beginning with Augustus, it was subsumed into the Imperial office.

Today, Pontifex Maximus is one of the titles of the Bishop of Rome as Roman Catholic Pope. As a papal title, the translation Supreme Pontiff is customary when writing in English, in which the Latin term Pontifex Maximus refers to the former pagan Roman post. But Latin is still the official Vatican language, and the Latin form Pontifex Maximus is still used in reference to the Pope when writing or speaking in that language.

In the Roman Republic, the Pontifex Maximus was the highest office in the polytheistic Roman religion, which was very much a state cult. His was the most important of the Pontifices (plural of Pontifex), positions in the main sacred college (Collegium Pontificum), which he directed.

The Pontifex was not simply a priest. He had both political and religious authority. It is not clear which of the two came first or had the most importance. In practice, particularly during the late Republic, the office of Pontifex Maximus was generally held by a member of a politically prominent family, since Augustus the emperor. Being Pontifex Maximus was not a full-time job and did not preclude the office-holder from holding a magistracy or serving in the military.

The Pontifices were in charge of the Roman calendar and determined when intercalary days needed to be added to sync the calendar to the seasons. Since the Pontifices were often politicians, and because a Roman magistrate's term of office corresponded with a calendar year, this power was prone to abuse: a Pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power. It was under his authority as Pontifex Maximus that Julius Caesar introduced the calendar reform that created the Julian calendar, with a fault under a day per century, easily corrected by a modification of the rules for bissextile days to produce our present Gregorian calendar.

In Christian circles, Tertullian furiously applied the term to Pope Callixtus I, with whom he was at odds, around 220 A.D. Was Pontifex a word in common currency by early 3rd century Christianity to denote a bishop? Tertullian's usage is unusual in that most of the technical terms of Roman paganism were avoided in the vocabulary of Christian Latin in favor of neologisms or Greek words. At the end of the 6th century Gregory I was the first Pope to employ Pontifex Maximus in a formal sense, in a broader program of asserting Roman primacy. It has remained one of the titles of the popes to this day.