  The class forcing theorem, which asserts that every class forcing notion
$\mathbb{P}$ admits a forcing relation $\Vdash_{\mathbb{P}}$, that is, a
relation satisfying the forcing relation recursion -- it follows that
statements true in the corresponding forcing extensions are forced and forced
statements are true -- is equivalent over G\"odel-Bernays set theory GBC to the
principle of elementary transfinite recursion $\text{ETR}_{\text{Ord}}$ for
class recursions of length $\text{Ord}$. It is also equivalent to the existence
of truth predicates for the infinitary languages
$\mathcal{L}_{\text{Ord},\omega}(\in,A)$, allowing any class parameter $A$; to
the existence of truth predicates for the language
$\mathcal{L}_{\text{Ord},\text{Ord}}(\in,A)$; to the existence of
$\text{Ord}$-iterated truth predicates for first-order set theory
$\mathcal{L}_{\omega,\omega}(\in,A)$; to the assertion that every separative
class partial order $\mathbb{P}$ has a set-complete class Boolean completion;
to a class-join separation principle; and to the principle of determinacy for
clopen class games of rank at most $\text{Ord}+1$. Unlike set forcing, if every
class forcing notion $\mathbb{P}$ has a forcing relation merely for atomic
formulas, then every such $\mathbb{P}$ has a uniform forcing relation
applicable simultaneously to all formulas. Our results situate the class
forcing theorem in the rich hierarchy of theories between GBC and Kelley-Morse
set theory KM.
