Why did the Bishop of Rome accept (and retains until today) the title "Pontifex Maximus" and how were the rights and duties transferred/transformed? According to the annual yearbook of the Catholic Church, the Bishop of Rome holds many other titles:

Vicar of Jesus Christ, Successor of the Prince of the Apostles, Supreme Pontiff of the Universal Church, Patriarch of the West, Primate of Italy, Metropolitan Archbishop of the Province of Rome, Sovereign of the State of Vatican City, Servant of the Servants of God.

The title Supreme Pontiff originated from a title usually held by the Roman Emperor, pontifex maximus (chief priest), whose main duty was to maintain the pax deorum (peace of the gods) by regulating expiatory sacrifices (to pagan gods) as well as administering the divine law (see wikipedia).
The first few Roman emperors who were Christian starting with Emperor Constantine still held this title, converting the title by imperial law (not church law) to make them the Head of the Church (giving Constantine the authority call the Council of Nicaea in 325 AD to settle the Arian controversy!), until Emperor Gratian gave the title to Pope Damasus I. (Source: The Title Pontifex Maximus).
Since it was a title having to do with official state religion, what was the reason that Pope Damasus accepted (and the subsequent bishops of Rome retained) the title &quot;Pontifex Maximus&quot; and how were the rights &amp; duties transferred / transformed?  Related subquestions (to help form the answer only):

Why wasn't the title retired instead, since &quot;Vicar of Jesus Christ&quot; is the title derived from the authority given by Jesus?

What is the significance of the title today (see picture of the inscription &quot;Benedict XVI, Pontifex Maximus&quot;)? What are the rights the current Pope enjoys and the duties that he needs to perform under this title that is not already covered by other titles?

When did the title become &quot;Supreme Pontiff of the Universal Church&quot; and what was the significance?

What does the Canon Law say about it?

Since the title originated when the Roman empire used to have an official religion (which became Christianity starting in AD 380) what is the relevance of the title in today's systems of secular governments, especially those holding the doctrine of separation of church and state?

If the title became useful as a state legal authority (as the article referenced above implies) can this title then be construed as potential interference with secular governments now that the age of Christendom has ceased?


Accepted answer should address the bold question at the minimum.
It is worth to describe in some detail that the title &quot;Pontifex&quot; pre-dates Christianity. According to Livius, the title was originally for (elected) high priests of the Roman religion, as mentioned in the Lex Ogulnia (300/299 BCE). The pontifex maximus was 'the greatest' or chairman of the college of the pontifices, 'priests'. They were responsible for the Roman state cult as a whole and for several cults in particular, namely the cults that had no priestly college of their own (such as the augures, the decemviri sacris faciundis and the fetiales).
In the period relevant to early Christianity, Julius Caesar was elected pontifex maximus in 63 BCE and kept the office until his death. After his death, Marcus Aemilius Lepidus became pontifex maximus (44-12 BCE); when he died, the emperor Augustus became responsible for the state cult. He also put an end to the election of the pontifices. From then on, a position in the college of pontifices was a sign of special imperial favor, comparable to a decoration in our age.
The word pontifex is sometimes explained as 'bridge builder', but is in fact related to the Etruscan word pont, 'road', and means something like 'preparer of the road'.
It should also be noted that Roman Emperors continued to bear the title Pontifex Maximus well beyond the time of establishment of the papal hierarchy (as in: the ultimate religious power of Rome was still with the emperor, not the pope). In fact, it was carried by all subsequent emperors until emperor Gratianus declined the title in 381. So the pope bearing the title pontifex maximus was not part of early Christianity. Only after Gratianus' refusal of the  title was it attributed to the pope (note that this is half a century after Christinanity became a state religion of Rome under Constantine in 313). Constantine himself actually held the title. It is frequently stated that Constantine made Christianity the only permitted religion in the empire: this is not the case, and it is also very likely the reason why Constantine did not relinquish the title to then pope Sylvester.
As to why the popes attributed themselves this title in stead of &quot;vicar of Christ&quot;, the most obvious worldly explanation is that the title had been attributed with the highest worldly + religious title in the empire for more than 600 years.
As to what significance &amp; privileges the title holds in today's world, I would say very little. But in the days of the empire, the Pontifex Maximus enjoyed many privileges, such as addressing the Roman Senate at will, the supervision of religious funds (of Rome, not the Vatican), authority over all public and private religious institutions, instruction of the populace in the celestial and funerary rites including appeasing the dead, and expiation of prodigies. He who held the title had political as well as religious power in Rome.
As for when did the title become &quot;Supreme Pontiff of the Universal Church&quot;, the word Catholic is derived via Late Latin catholicus, from the Greek adjective καθολικός (katholikos), meaning &quot;universal&quot;.