You missed where he explained that the bible is and can be the sole source of Christian truth. You might want to ask about this.
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.