Kairos refers to the timing in which a rhetorical text is made. This can mean when an argument is presented during the most pertinent time possible, such as Dr. Martin Luther King Jr. presenting his "I Have a Dream" speech during the height of the Civil Rights Movement. The kairos of the film Elmer Gantry is largely derived from the eponymous novel of which it is adapted from.
The 1927 novel Elmer Gantry was based on the career/actions of Aimee Semple McPherson. McPherson was the founder of the Pentecostal Christian movement known as Foursquare Church. As stated previously, this was around the era of American history where there was a sharp advent of religious movements. The author Sinclair Lewis likely wrote his book to satirize such. He also wrote literature critical to capitalism and the wars taking place in his time. It is also good to note that McPherson was a critic of communism, as she believed it had the goal of "ruling without God," while also opposing fascism. The controversial movements introduced in the times of both the novel and the film were the driving force in their creation.