It must be understood that it took THE CHURCH to determine what books were to be in the Bible in the first place, since there were a very large amount of Scriptures being circulated early on in the Church. By 393AD at the Council of Hippo,THE CHURCH defined the canon, which is exactly the same list of books that were defined at Trent. Also a few years after Hippo at the COuncil of Carthage, St. Augustine and the Church defined the canon again with the same list of books, and there were many other councils that did the same thing after Carthage. The Council of Trent gave the final definition so that it would never be an issue and so it would be binding for good.
So I would say the real question to ask someone who does not believe in the authority of the Catholic Church is this, “Why do you believe any of the other books of the Bible to be Canon? What about all the other books such as Clement, Hermas, Barnabas, etc that did not make it into the canon? Why would you leave them out, these men’s names are in the Bible as fellow workers of the Apostles? Most Protestants don’t even realize that so many other writings exist, and even if some of them do they have never studied them enough to give an educated answer to why these books are not good enough to be canon, they just rely on some Pseudo-‘scholar’ who give a very biased reformation minded opinion. Also, if the Bible is there only authority, then who had the authority to determine what books are canon in the first place?”
As for the Greek Septuagint, I know from personal research of my own that at least 85% of the NT quotations of the OT come straight from the Septuagint verbatum. I have experience as a translator and in textual criticism, and I am very familar with the Greek NT and the LXX (Septuagint). And if the LXX was good enough for the authors of the NT then it is good enough for me! All the early Apostolic Fathers quoted the LXX, such as St. Clement, St. Barnabas, St. Hermas, St. Justin, St. Irenaeus, the author of the Didache, etc.
Also among the Dead Sea Scrolls you will find Tobit, Sirach in Aramaic and Hebrew!
One thing you will find among Protestants concerning the Bible is that for many of them it has become to them a religion of a Book, the 66 books of the Protestant Bible that is. Many of them are very confused about where the Bible comes from and they just take a ‘leap of blind faith.’ They love the Bible, and their zeal is to be admired by Catholics, and alot of Catholics have alot to learn from there zeal. But the system of sola scriptura is flawed with many weak spots. And many protestant sects rely totally upon that system. Most of the followers love Jesus with all there hearts and do make it to Heaven, but the system leads to theological anarchy and confusion, which is the reason why they accept just part of the Word of God and miss out on the fulness of the Faith.