Your argument has a salvation component to it. All knowledge for salvation is in John’s Gospel. It says it is so. So if you put forth another salvation requirement, Marian doctrines, you operate outside of scripture and preach a different gospel than the Bible does. Even angels doing so are accursed. Gal 1:8. So that goes for non-angels as well.
Where does that leave the Christians who were always “outside of Scripture”?
How do you know Scripture is important?
For doing what? For departing from Scripture? I don't think so.