Fides was the goddess of sworn faith — of the keeping of oaths, of the binding of one's word, of the trustworthiness on which all human dealings (commerce, treaties, political alliances, marriage, friendship, master-and-slave) depended. Her cult was ancient, attributed by tradition to King Numa, the second king of Rome (whom the Roman antiquarian tradition credited with the foundation of most of the state religion). She had a temple on the Capitoline hill, near that of Jupiter, with whose oversight her function was closely connected. The most distinctive feature of her cult was the priestly observance: the three major flamines (those of Jupiter, Mars, and Quirinus) drove together in a chariot to her temple to make sacrifice, and at the rite they covered their right hands in white wool — the right hand being the hand of the oath, the hand whose grip in agreement was the physical seal of fidelity, which was thus presented to the goddess in a state of ritual veiling that emphasized its sacred function.

Fides was one of the most ancient and central of the Roman virtues, and her deification reflected the Roman conception of the binding power of the spoken word. The phrase Fides publica meant the credit of the state — the sworn word of Rome to its allies and treaty-partners, on which the entire Roman foreign policy was held to rest (or to depend on the appearance of resting). The Romans were, by the testimony of their friends and enemies, often genuinely scrupulous about treaty obligations; Polybius the Greek historian, who was a hostage at Rome and had ample opportunity to observe, attributed Roman success in part to a real cultivation of fides as a state virtue, though he was not blind to the cases where it was breached. The deification of fides, in any case, was the religious expression of this national self-conception.