I have at home a King James version of the Bible with the deutero-canonical books inserted between the Old Testament and the New Testament. When people like your friend come to visit, I frequently ask them which parts of the Bible are the inspired Word of God. They usually say that all the books in the King James bible are the inspired Word of God. I then point out that my King James bible includes Tobit, Judith, 1 and 2 Machabees, portions of Esther and Daniel, Baruch, Ecclesiasticus, and Wisdom of Solomon. Why, I ask them, don’t they accept those books as canonical? And why not include the Gospel of Pilate? or Thomas? or Peter?
You may not have a King James version of the Bible with the deutero-canonical books inserted between the Testaments, but you probably get my point. The whole (Sacred Tradition) is greater than the part (Sacred Scripture). The Tradition that tells us which books belong in the Bible and which books don’t belong in the Bible is the same Tradition that tells us the Blessed Virgin was assumed into Heaven, St. Peter was the first bishop of Rome, etc. The Bible faithfully reflects part of that Tradition, but as St. John observes, he and the other Apostles and Disciples did not write down everything Jesus Christ did or taught. Does that mean Christians aren’t obligated to follow the things Jesus Christ taught that aren’t written down in the Bible? God forbid!
We know what we must believe through the Catholic Church, which Jesus Christ promised to guide until the end of time. How then could the Catholic Church every go astray when it has as its guide He who is the Way, the Truth, and the Life?