Having in this way obtained the crown, Numa prepared to found, as it were, anew, by laws and customs, that City which had so recently been founded by force of arms. He saw that this was impossible whilst a state of war lasted, for war brutalized men.
Thinking that the ferocity of his subjects might be mitigated by the disuse of arms, he built the temple of Janus at the foot of the Aventine as an index of peace and war, to signify when it was open that the State was under arms, and when it was shut that all the surrounding nations were at peace…
After forming treaties of alliance with all his neighbours and closing the temple of Janus, Numa turned his attention to domestic matters. The removal of all danger from without would induce his subjects to luxuriate in idleness, as they would be no longer restrained by the fear of an enemy or by military discipline. To prevent this, he strove to inculcate in their minds the fear of the gods, regarding this as the most powerful influence which could act upon an uncivilized and, in those ages, a barbarous people.
But, as this would fail to make a deep impression without some claim to supernatural wisdom, he pretended that he had nocturnal interviews with the nymph Egeria: that it was on her advice that he was instituting the ritual most acceptable to the gods and appointing for each deity his own special priests.
…
Next he turned his attention to the appointment of priests. He himself, however, conducted a great many religious services, especially those which belong to the Flamen of Jupiter. But he thought that in a warlike state there would be more kings of the type of Romulus than of Numa who would take the field in person. To guard, therefore, against the sacrificial rites which the king performed being interrupted, he appointed a Flamen as perpetual priest to Jupiter, and ordered that he should wear a distinctive dress and sit in the royal curule chair. He appointed two additional Flamens, one for Mars, the other for Quirinus, and also chose virgins as priestesses to Vesta.
Here was laid down with what victims, on what days, and at what temples the various sacrifices were to be offered, and from what sources the expenses connected with them were to be defrayed. He placed all other sacred functions, both public and private, under the supervision of the Pontifex, in order that there might be an authority for the people to consult, and so all trouble and confusion arising through foreign rites being adopted and their ancestral ones neglected might be avoided.
Nor were his functions confined to directing the worship of the celestial gods; he was to instruct the people how to conduct funerals and appease the spirits of the departed, and what prodigies sent by lightning or in any other way were to be attended to and expiated. To elicit these signs of the divine will, he dedicated an altar to Jupiter Elicius on the Aventine, and consulted the god through auguries, as to which prodigies were to receive attention.
(Livy, Ab Urbe Condita, text from the Perseus Digital Library)