They are "defending" an alleged "Christian flag" not recognized by most Christians.
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You are displaying gross ignorance and/or religious bigotry.
Go to any image search engine, and search for Christian Flag. What you will see is a full screen of exactly the same flag that is flying from those pickup trucks.
(Be forewarned: on Bing Images, the page includes a single example of an abomination of perversion -- a mockery of the Christian flag -- with the white replaced by rainbow-stripes and a cross that is feces-brown...)
Otherwise, you will find that this
is recognized by the majority of Americans (including a majority of Christians) -- as well as the companies who make and sell flags -- as "The Christian Flag".
“They are ‘defending’ an alleged ‘Christian flag’ not recognized by most Christians.”
Really? “Alleged”? You need to get out more. Most (true) Christian churches have the Christian Flag in the church. I learned the Pledge To The Christian Flag when I learned the Pledge to the American Flag.