Full Idea
A proof of the consistency of propositional logic was given by Emil Post in 1921.
Gist of Idea
Post proved the consistency of propositional logic in 1921
Source
Michal Walicki (Introduction to Mathematical Logic [2012], History E.2.1)
Book Reference
Walicki,Michal: 'Introduction to Mathematical Logic' [World Scientific 2012], p.35