It seems strange to have to say that Catholics are Christian but to a growing number of Evangelical Protestants (particularly in the Anglosphere) it needs to be pointed out. Catholic Christianity is Complete Christianity. In fact, we Catholics were the first Christians because we Catholics were the original Christians. Sadly, a lot of Catholics have apparently played into this word-trap as well, referring to Catholics as “Catholics” and to Protestants as “Christians.” What they’re doing, inadvertently, is giving credit to the false notion that Catholics are not Christians and Protestants are. It’s a terrible situation that currently exists, and this blog is aimed at putting an end to it — forever.
You see, the word Catholic simply means “universal” or “all-embracing” or “including a wide variety of things.” In other words “complete.” It was used in the late first century to describe the actual Church established by Christ and his apostles, which spanned the ancient world, in contrast to many sectarian groups which only followed the teachings of a specific leader or were limited in membership to geography or ethnicity.
In modern times, within the last 200 years or so, groups of Protestants began breaking away from their mainline denominations and national churches, and a good number of them began calling themselves Evangelical. Later, they began using the terms “nondenominational” or “Born-Again” or “Bible Christians.” Some of them insisted on just calling themselves “Christians” without any adjective to describe or define them. In time these non-descriptive Evangelicals started using the word “Christian” to compare/contrast themselves with other Christians using different names. A large number of these non-descriptive Evangelicals referred to themselves as “Christian” and to members of the Catholic Church simply as “Catholics” as if these two words (Christian and Catholic) has nothing in common at all. Thus, in great swaths of Anglophone societies, many Evangelical Christians do not believe that Catholics are Christian. This is ridiculous!
— Shane Schaetzel, Complete Christianity, Are Catholics Christian? May 12, 2019