A note on historicity
Numa belongs to the regal period the Romans themselves regarded as shading into legend. Whether he is a single historical king or a figure to whom the early Roman religious and civic institutions were later attributed is debated; modern Roman historians read him with the same care they give to Lycurgus. Livy's Ab Urbe Condita Book I and Plutarch's parallel Lives of Numa and Lycurgus are the main ancient sources.
What the Roman tradition credits him with
The Roman tradition reads Numa as the institutional founder of the city's religious and civic order — the deliberate counter to the warrior reign of Romulus. He is credited with establishing the major priesthoods (the pontifices, the flamines, the Vestal Virgins), with regulating the Roman calendar, with consecrating the temple of Janus whose gates were closed during peace, and with the long set of practices by which Roman public life would be ordered through ritual oath, augury and the careful observation of sacred precedent.
Plutarch frames the contrast with Romulus directly: Romulus founded Rome by force; Numa preserved it by custom and law.
Why he matters for Virtue & Power
Numa is the Roman case for the lawgiver-founder who works through institutions rather than through arms. The classical pairing with Lycurgus is deliberate: Plutarch wrote them as parallel Lives, and the early-modern republican tradition read them as the two great archaic examples of founding through law rather than through conquest. The platform places him in the Founding theme alongside Lycurgus and Solon. He is also one reason the Roman political imagination treated civic virtue as historically continuous with religious practice rather than as a separable thing.