Shane Steven, the writer of the Catholic in the Ozarks blog, wrote an insightful and personal piece about how some fundamentalist Christian faithful that he has encountered in the Ozarks consider Catholics to be in a cult and hence not Christian.
The gist is that the way they apply sola scriptura to the practice of faith makes it logical for them to come to that conclusion. Stevens does a yeoman's job of dispelling this myth through a historical and rational apologetic.
For those curious about comparative theology and understanding who is part of the Christian brethren, this piece "Are Catholics Christians?" is highly commended.