It's Not Just Modern Evangelicals

Some traditional fundamentalists have had the same belief. It really goes back to the Reformation, although it might have been expressed differently.

The rationale is that the Christian gospel involves salvation by faith in Jesus alone, which is spelled out in Paul's letter. Catholicism, ooh, teaches a different gospel, one in which good works and obedience to the church are important, and amounts to little more than Christianized paganism. Reformation-oriented Protestants hold that believing in sola fides and sola scriptora (only faith and only scripture) are essential to true Christianity.

There is a whole host of associated criticisms of the Catholic church which highlight the stranger elements and those that do not seem to be in keeping with specific Bible passages. These includes having a pope and a hierarchy, praying to Mary and the saints, and placing church tradition on a par with the Bible. Some of these are surprisingly similar to anti-historical-Jesus arguments such as incorporating elements of Mithraism into the church.

There is a whole spectrum of attitudes towards Catholicism. Simply dismissing it as a non-Christian religion is kind of an extreme one, but it's prevalent. If you live in an area of the country with no Catholics, you can go years without having your views disabused.