fides

Fides, fidei is fifth declension feminine, one of the very few fifth-declension nouns you meet often, along with res and dies. Genitive and dative are both fidei, and it has no plural in practice.

The Roman sense runs in both directions at once: fides is the trust you place in somebody and the trustworthiness you show them. That two-way quality is why it covers credit in banking, protection in diplomacy and a guarantee in law. In fidem venire is to surrender into somebody's good faith; fidem dare is to give your word; bona fide, in the ablative, is the phrase English simply borrowed. The trap is the Christian corpus, which narrows fides hard to faith as belief, and reading that back into Cicero will mislead you badly. Family: fido, confido, fidelis, perfidia, foedus (rank 701). English fidelity, confidence, infidel, perfidy, affidavit; French foi, Spanish fe.