It must be addressed by both Catholics and Protestants and is often overlooked in the arguments over the "when, where, what, by who" of books being added/used/confirmed by various scholars, both Jewish and Christian.
Yes, the debates will get that detailed. (I know, you're breathless with anticipation.)
This fact is: we have to decide which books are canonical somehow. The Bible does not - cannot - do so on its own. So we (all Christians: Catholics and Protestants) must look to both authority and criteria. But which authority do we choose? Christians disagree. For now, let's look at the criteria for what is canon. And what it is not.
The word "canon" comes from the Greek "κανών", meaning "rule" or "measuring stick". The concept is believed to have predated Christ, though. First we'll look at a couple of criteria that are often cited, but are invalid.
• Is it divinely inspired?
No book of the Bible claims itself to be divinely inspired. Jesus never left a list of Old Testament books to be followed (nor a list of New Testament books…which would be out of order choronologically…but not impossible for God). "The term 'inspired' (Greek, theopneustos) only occurs once in the Bible (2 Tm 3:16), where we are told that all Scripture is inspired.
We first know that something is Scripture and then infer that it is inspired; we do not first know that it is inspired and then conclude it is Scripture" (Catholic Answers, ref. 1). Now, other sections of the Bible may indicate previous sections were inspired (by saying "God spoke" or "the Spirit spoke"), but we must acknowledge that something can claim to be inspired, but not actually be so. (The Book of Mormon, for example).
• Does it agree with doctrine?
This is a circular argument. Some people don't like the idea of praying for the dead, for example, so they point to 2 Maccabbees and say "it isn't canon." Here is the thing, though: just because someone doesn't personally like a practice or doctrine in the Bible doesn't mean it shouldn't be in the Bible. We get our doctrines and beliefs from the Bible; not the other way around.
However, there is some merit to the argument. If doctrine clearly disagrees with another section of the Bible (a section which is generally deemed to be of high authority), then we can start to consider it a valid argument.
Even then, we have to realize this is a complex argument. James 2:24 and Romans 4:3 (accepted by all Christians as canonical) teach different things: salvation by faith or by faith plus works? Many things appear contradictory in scripture; Christians attempt to reconcile these things all of the time.
The following are canon requirements that are generally agreed upon, but still with reservations. The debate comes in when we start arguing over who/what/when.
• Was the author an apostle or have a close connection with an apostle? Or in the OT, what the author a prophet or have a close connection with a prophet?
The argument comes in as to who "qualifies" as a prophet or apostle and what defines as "closely connected" with them. We also have the slight problem of Esther, who wasn't a prophet, but the book is universally accepted. And the Book of Wisdom, not accepted by Protestants, has prophecy in it.
• Did the book bear evidence of high moral and spiritual values that would reflect a work of the Holy Spirit? Does it "ring true" with the reader? (Sometimes states as the book "transforms" the reader).
This is almost entirely subjective, thus while it makes sense to have as criteria, it usually does not lead to consensus among Christians.
• Is the book being accepted by the body of Christ at large? (There will always be individuals who do not agree; what do Christians as a whole- especially those who are learned- think?)
This is where the debate comes in, as well as the "we must accept authority." We will address these questions- at length- below. Along with discussions of what various individuals thought. There is an entire (in progress) page dedicated to lists of canon at various times. The major ones will be listed at the top in chronological order, with the others listed alphabetically. See: "Canon Lists Throughout History" page for this.
Thus, after looking at this subject, we find that the Bible does not (and can not) state what is Scripture. More bluntly, the Bible didn't come with a table of contents. And Jesus didn't leave us "to read" list of books. Thus, we will turn to authority. Our best authorities are those closest to Christ. So…we shall look to history.
****Note: The material for these posts will probably end up filling many pages when I'm done, but I'll be posting as I go along in "quick bites" for people who are interested in such things. But this means that I won't have read everything or know everything (if such a thing is possible) when I write each blog post. I'm learning as I continue to research. Should you find an error, please let me know! Please, please, reference "Canon History" for the most up-to-date, accurate information!