Formula-inaccessible cardinals and a characterization of all natural models of Zermelo–Fraenkel set theory

E. Zermelo (1930) and J. C. Sheperdson (1952) proved that a cumulative set $V_\alpha$ is a standard model of von Neumann–Bernays–Gödel set theory if and only if $\alpha=\varkappa+1$ for some inaccessible cardinal number $\varkappa$. The problem of a canonical form for all natural models of ZF theory turned out to be more complicated. Since the notion of a model of ZF theory cannot be defined by a finite set of formulae, we introduce a new notion of (strongly) formula-inaccessible cardinal number $\theta$ using a schema of formulae and its relativization on the set $V_\theta$, and prove a formula-analogue of the Zermelo–Sheperdson theorem giving a canonical form for all natural models of ZF theory.