Roman historical tradition, recorded most fully by Livy and Plutarch centuries after the events it describes, credits Numa Pompilius with an extraordinary act of religious institution building during a reign conventionally dated to the late eighth and early seventh centuries BCE. Elected king by the Roman senate after a deliberate year long interregnum following Romulus's disappearance, Numa is said to have found a city skilled in war and lacking in ordered worship, and to have set about supplying exactly that. Tradition assigns him the founding of the pontifical college, charged with overseeing Roman religious law generally, the augurs, who read the will of the gods in the flight of birds, the flamens, priests dedicated individually to particular gods such as Jupiter, Mars and Quirinus, and the Vestal Virgins, who tended Rome's sacred hearth fire in the temple of Vesta and whose perpetual virginity was treated as bound up with the city's own safety.
Numa is also credited with building the temple of Janus, whose doors were closed only in times of full peace, and with reforming the Roman calendar from ten months to twelve by adding January and February, bringing it closer into line with the solar year. Ancient sources disagree on how far to credit an actual historical Numa with all of this institutional invention, but the consistent thread across the tradition is that whatever existed of Roman public religion by the historical period was retrojected onto his reign as its founding moment.