Full Idea
Löwenheim's theorem reads as follows: If a first-order proposition is satisfied in any domain at all, it is already satisfied in a denumerably infinite domain.
Gist of Idea
If a 1st-order proposition is satisfied, it is satisfied in a denumerably infinite domain
Source
Thoralf Skolem (Remarks on axiomatised set theory [1922], p.293)
Book Reference
'From Frege to Gödel 1879-1931', ed/tr. Heijenoort,Jean van [Harvard 1967], p.293