Any reading of 18th, 19th and early 20th century American literature would quickly force this position to retreat. You will find it quite common for a protagonist or narrator to describe themselves as quote "Christian" w/o any qualifier.
Further, the election of 1884, which featured the virulent anti-catholic James Blaine represents one of the few Rep. defeats of the later 19th century. This in a nation overwhelmingly protestant.
And so the entire premise is contingent upon the ignorance of his audience. How unfortunate that that audience is as large as it is.
According to the Bible the term “Christian” was applied to believers in Jesus at Antioch in the first century b.c. Doesn’t sound very recent to me.