Let $I$ be the category of finite sets and monomorphisms.
The functor $P:I\rightarrow Sets$ is a sheaf for the atomic topology on $I^{op}$ iff $P$sends every morphism of $I$ to a monomorphism and $P$ preserves pullbacks.
I have no idea how to prove this... I tried to show the pull back of a monomorphism is itself a monomorphism by the universal property...