Lemma 6.9.2. Suppose the category \mathcal{C} and the functor F : \mathcal{C} \to \textit{Sets} have the following properties:
F is faithful,
\mathcal{C} has limits and F commutes with them, and
the functor F reflects isomorphisms.
Let X be a topological space. Let \mathcal{F} be a presheaf with values in \mathcal{C}. Then \mathcal{F} is a sheaf if and only if the underlying presheaf of sets is a sheaf.
Proof.
Assume that \mathcal{F} is a sheaf. Then \mathcal{F}(U) is the equalizer of the diagram above and by assumption we see F(\mathcal{F}(U)) is the equalizer of the corresponding diagram of sets. Hence F(\mathcal{F}) is a sheaf of sets.
Assume that F(\mathcal{F}) is a sheaf. Let E \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}) be the equalizer of the two parallel arrows in Definition 6.9.1. We get a canonical morphism \mathcal{F}(U) \to E, simply because \mathcal{F} is a presheaf. By assumption, the induced map F(\mathcal{F}(U)) \to F(E) is an isomorphism, because F(E) is the equalizer of the corresponding diagram of sets. Hence we see \mathcal{F}(U) \to E is an isomorphism by condition (3) of the lemma.
\square
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