 Monic endomorphism of a subobject classifier

In a category with finite limits and a subobject classifier, every monic endomorphism $f$ of $\Omega$ is an involution. The subobject $g:U\hookrightarrow \Omega$ classified by $f$ is subterminal, since $fg=\top {!}_U$ is monic. If $V\hookrightarrow U$ is classified by $g$, pullback uniqueness gives $f\top {!}_U=g$, hence $f^{2}g=g$. If $u$ classifies $U\hookrightarrow 1$, the pullback of $g$ along $f$ is $\top {!}_U$, so $f^2(p)=u\wedge p$. Monicity implies $u=\top$, because $u\wedge u=u\wedge \top$, proving the displayed identity.