In the film, Rick discovers that he can use his war room to magically make his prayers come to life, using this new power to cause Samuel, the park ranger and everybody that disagrees with him to die by bursting into flames. The next day, Kendrick appears at his door and proposes a movie deal with him. Even with the satirical nature of the film, Kendrick seems too suspiciously calm and unaffected by his twin brothers sudden demise. Not only does he not seem be bothered by Samuels death, but he then proceeds to cash in on a business opportunity with the guy who confessed to killing him online. Even worse, it is Kendrick offering Rick his movie deal that triggers the end of the world. The film parodies the idea that "Christians are the only good people" commonly found in Christploitation, and yet Kendrick is the only Christian other than Rick who seems to be motivated by selfish reasons. Could it be possible that Kendrick isnt as sincere as he would want you to believe?
The Devil is established as a character that thrives off of the chaos and suffering of mankind. While Santa Christ claims that he can see through space and time, he is clearly not omniscient, legitimately horrified and surprised by how far the bet has gotten (mass death, apocalypse, etc). It stands to reason that the Devil was manipulating Santa Christ with the intent of purposely starting the Rapture, with all of the non-white male Christian demographic staying behind to be tormented during the tribulation as a bonus and behaving as though he did not intend to cause it as a trump card in case Santa Christ somehow fixed the situation himself.
An even more absurd idea, there never was a "Kendrick", but instead he was actually Samuel, sent by the Devil after he died and went to Hell to kickstart the apocalypse and have Rick and Santa Christ take the blame for it when God arrives.