Ultrafilter Lemma

Theorem
Let $X$ be a set.

Every filter on $X$ is contained in an ultrafilter on $X$.

Proof
Let $\Omega$ be the set of filters on $X$.

From Subset Relation is Ordering, the subset relation "$\subseteq$" makes $\Omega$ a partially ordered set.

If $C \subseteq \Omega$ is a non-empty chain, then $\bigcup C$ is again a filter on $X$ and thus an upper bound of $C$.

For any $\mathcal F \in \Omega$ there is therefore by Zorn's Lemma a maximal element $\mathcal F'$ such that $\mathcal F \subseteq \mathcal F'$.

The maximality of $\mathcal F'$ is in this context equivalent to $\mathcal F'$ being an ultrafilter.

Comment
Note that this result, dependent as it is upon Zorn's Lemma, is a consequence of the Axiom of Choice.