Full Idea
Before Tarski's work in the 1930s, the main results in model theory were the Löwenheim-Skolem Theorem, and Gödel's establishment in 1929 of the completeness of the axioms and rules for the classical first-order predicate (or quantificational) calculus.
Gist of Idea
Löwenheim-Skolem Theorem, and Gödel's completeness of first-order logic, the earliest model theory
Source
Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V)
Book Reference
Feferman,S/Feferman,A.B.: 'Alfred Tarski: life and logic' [CUP 2008], p.281