Finitely Satisfiable Theory has Maximal Finitely Satisfiable Extension/Proof 1

Proof
The set of all finitely satisfiable $\mathcal L$-theories containing $T$ forms an ordered set using subset inclusion as the ordering.

Let $C$ be a nonempty chain in this ordered set.

Let $\displaystyle T_C = \bigcup_{\Sigma \mathop \in C} \Sigma$.

Let $\Delta$ be a finite subset of $T_C$.

Then there exists a single $\Sigma$ in $C$ which contains $\Delta$.

Since this $\Sigma$ is finitely satisfiable by definition, this means that $\Delta$ is satisfiable.

Hence $T_C$ is finitely satisfiable.

Since each $\Sigma \in C$ is contained in $T_C$, this means that $T_C$ is an upper bound for $C$ in the ordered set.

Thus, by Zorn's Lemma, there is a finitely satisfiable $\mathcal L$-theory $T'$ containing $T$ such that $T'$ contains all other such theories.

Let $\phi$ be an $\mathcal L$-sentence.

Let $\phi \nsubseteq T'$.

Aiming for a contradiction, suppose $T' \cup \left\{ {\phi}\right\}$ were finitely satisfiable.

Then by definition of $T'$, $T'$ would contain $T' \cup \left\{ {\phi}\right\}$ as a subset.

This would mean that $T'$ contains $\phi$, which contradicts the assumption.

Thus $T' \cup \left\{ {\phi}\right\}$ is not finitely satisfiable.

By the lemma, $T' \cup \left\{ {\neg \phi}\right\}$ is finitely satisfiable.

Thus, by definition of $T'$, $T'$ contains $T' \cup \left\{{\neg \phi}\right\}$ as a subset.

Hence $T'$ contains $\neg \phi$.