From Elected Priesthood To Imperial Title

For most of the Republic, the pontifex maximus was chosen by co-optation, meaning the existing College of Pontiffs selected new members and its own head from among Rome's elite; a law of 104 BCE, the lex Domitia, shifted selection to election by a specially convened subset of the Roman citizen assembly, a change that opened the office more directly to political competition. Julius Caesar won election as pontifex maximus in 63 BCE, a position he held alongside his subsequent political and military career and which contributed to his religious as well as political authority in the years before his assassination in 44 BCE.

Under the Empire, Augustus took the office himself in 12 BCE, and from that point the title passed automatically to whoever held imperial power, fusing supreme religious and political authority in the person of the emperor for the following four centuries. The office lapsed as a state institution when the Christian emperor Gratian declined the title around 375 to 382 CE, a symbolic break with Rome's traditional state cult as the empire moved toward Christianity as its official religion. The Latin title was later adopted by the Bishop of Rome as one of several papal titles, a deliberate act of appropriated Roman vocabulary rather than any institutional continuity from the pagan Roman priesthood.