All Christians believe this. As a Roman Catholic Christian (I despise all the labels) I believe this as all Roman Catholics do. The question to ask yourself is: Did Paul believe that all the letters that he wrote were sacred scripture? Did Paul read Revelation? Did he approve? The question about several of the "Catholic" epistles even being scripture (Luther had a problem with them) is another argument that has perplexed the 2 Timothy 3:16 quote.
One absolutely has to refer to Church authority when it comes to what is scripture and what is not. And that is what Arthur was stating.
Deacon Francis
I was referring to the use of this verse by sola scriptura advocates. It would support their position if it said that ONLY Scripture contains the deposit of faith.
Scott Hahn was an adult, a student preparing for the ministry, when he first noticed that the verse said “ALL” rather than “ONLY.”