The “Catholics aren’t Christian” argument has stemmed from many, many misunderstanding sof the faith.
I think much of it goes back almost 500 years wgo with the start of the Reformation. Many people began to misinterpret Scripture, the role of authority, the Tradition, etc. People began to have their own ideas and they found followers.
As for where in Scripture these arguments come from – they aren’t in Scripture. It is some person’s misunderstanding of the truth that has misguided many, many people.
Personally, I think that the Bible alone movement was very detrimental. I find it very precarious that people think they can limit God’s interaction with humankind to a set of books (or one book as some think).
Heck, even the Bible proves that to be false. The Bible shows us that God has not revealed himself to us [i:1x8a53ae]only[/i:1x8a53ae] in the printed word, but has interacted with humans for a very long time. All the stories in the Bible show us this. There is no support for Scripture alone in Scripture, yet some people are so blind to this.