Material sufficiency of Scripture
In Part I, we saw how the basic premise of sola scriptura (only Scripture is infallible) must implicitly contradict Scriptural promises of either the Holy Spirit's guidance or the Holy Spirit's reliability to be true. In Part II, we saw how sola scriptura effectively and logically denies anyone the authority of the Spirit, including the Protestant defender of sola scriptura.
Part III addressed the question of human authority as it affects the composition of Scripture. Again, the contradiction at the heart of sola scriptura denies the infallible authority of the Spirit to determine the canons: If the Church could be wrong about including the Old Testament deuterocanonical books, then all Christian churches could be wrong about denying a role for the Gospel of Thomas or the Book of Moroni.
In this part, we finally start to address the Scriptural backing for it. If we remember, the first principle of sola scriptura is: Scripture, taken by itself, is sufficient to act as the regula fidei, the infallible rule of faith. Although we haven't addressed this point directly, this is the key claim; if it's not true, then nothing else that follows from it can be true.
Proof text 1: 2 Tim 3:14-17
All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the servant of God may be thoroughly equipped for every good work.
Now, if you're going to argue that Scripture is sufficient to act as the rule of faith, it would be helpful if your proof text said sufficient. But the word is ōphelimos—profitable, useful. A hammer is a jim-dandy tool to have, but it won't work for all fix-it projects. Moreover, it says "All Scripture is God-breathed" … not "Only Scripture is God-breathed"; it doesn't follow from this passage that nothing else can be inspired by God.
Proof text 2: John 20:30-31
Jesus performed many other signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name.
What is being written "so you may believe"? The signs (i.e. acts, especially miracles) Jesus performed. If we put the construction the Protestant wishes on this passage, then we'd be proving too much: all we need is the Gospel of John! Patently, though, John is simply recording some of Jesus' words and deeds to support beliefs the community already had, not to create that belief anew, and not to suggest that the community needed nothing else.
Proof text 3: Luke 10:25-28
On one occasion an expert in the law stood up to test Jesus. "Teacher," he asked, "what must I do to inherit eternal life?" "What is written in the Law?" he replied. "How do you read it?" He answered, "'Love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind' (Dt 6:5); and, 'Love your neighbor as yourself'"(Lev 19:18). "You have answered correctly," Jesus replied. "Do this and you will live."
This almost cries for a loud "A-HA! All we need for eternal life is Scripture! And Jesus himself said it!"
But let's think about that conclusion for a minute. If all we needed for salvation were the Law of Moses—because Jesus' question isn't, "What is written in the prophet Isaiah?" but, "What is written in the law?"—then, please tell me: What was the point of Jesus' death on the cross? The Jews already had the law! "But now we are discharged from the law, dead to that which held us captive, so that we serve not under the old written code but in the new life of the Spirit" (Rom 7:6). Talk about proving too much: Not only does such an interpretation distort Jesus' intent, it completely negates Christianity!
Let's look at the passage with clearer sight: The lawyer asks, "What must I do to gain eternal life?" This is a question of ethical behavior: not "What must I believe?" or "How must I worship?" but "How should I live?" Jesus made a similar statement to the rich young man: "If you want to enter life, keep the commandments" (Mt 19:18). And a person who conscientiously lives by the Ten Commandments, we can believe, has a good chance even if his faith is weak and erratic. Once the limits of both question and answer are made clear, we see that this passage doesn't reach sola scriptura.
Proof text 4: Lk 16:27-31
"[The rich man said], 'Then I beg you, father [Abraham], send Lazarus to my family, for I have five brothers. Let him warn them, so that they will not also come to this place of torment.' Abraham replied, 'They have Moses and the Prophets; let them listen to them.' 'No, father Abraham,' he said, 'but if someone from the dead goes to them, they will repent.' He said to him, 'If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead.'"
This text was actually quoted in support of sola scriptura … by an ex-Catholic yet! Not only is the line clearly out of context, it raises the same problem that proof text 2 raises: The Jews already had the Law and the Prophets. It supports St. Paul's contention that Scripture is useful for training in righteousness; however, it doesn't support the contention that Scripture is sufficient to act as sole regula fidei.
Proof text 5: 2 Peter 1:12-21
So I will always remind you of these things, even though you know them and are firmly established in the truth you now have. I think it is right to refresh your memory as long as I live in the tent of this body, because I know that I will soon put it aside, as our Lord Jesus Christ has made clear to me. And I will make every effort to see that after my departure you will always be able to remember these things.For we did not follow cleverly devised stories when we told you about the coming of our Lord Jesus Christ in power, but we were eyewitnesses of his majesty. He received honor and glory from God the Father when the voice came to him from the Majestic Glory, saying, "This is my Son, whom I love; with him I am well pleased." We ourselves heard this voice that came from heaven when we were with him on the sacred mountain.We also have the prophetic message as something completely reliable, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts. Above all, you must understand that For prophecy never had its origin in the human will, but prophets, though human, spoke from God as they were carried along by the Holy Spirit.
Oddly enough, both sides use the text against each other; to Protestants, Peter's promise (in bold black) is a foreshadowing of a materially sufficient New Testament; unfortunately, that isn't a necessary or inevitable deduction.
But the key text here is in bold red, where Peter says the apostles didn't "follow [the authority of] cleverly devised stories" (Gr. sesophismenois mythois exakolouthēsantes). Mythos can mean a story that's either true or false, though Thayer's Lexicon indicates that Peter had in mind Jewish theosophy and Gnosticism; Peter's point is that he and the apostles aren't telling a story somebody else wrote but rather delivering first-hand testimony.
Because of this personal witness, the "prophetic message" doesn't refer back to Scriptures that may not have been written yet; it refers to the Apostles' possession of the Holy Spirit which imparts reliability. And the bolded, highlighted text properly understood means that the prophets of the Old Testament didn't get their prophecies out of books. Far from supporting sola scriptura, the passage undermines it.
* * *
If nothing else, these citations demonstrate the dangers of cherry-picking proof texts. They certainly don't prove the material sufficiency of Scripture, or that the writers had a materially-sufficient New Testament in mind. In fact, given the low rates of literacy (estimated not higher than 10% for most of the Roman Empire), it would be a marvel if any of the apostles had foreseen such a repository.
If you can't prove that Scripture is materially sufficient to act as the regula fidei, then you can't hold as a principle that it contains everything a Christian must believe, or that he can't be bound to accept what it doesn't contain. However, when we come back to this issue, we'll see how the problem of material insufficiency ties into the problem of Scripture's formal insufficiency.
- Scripture itself doesn’t support the material sufficiency of Scripture to act as the sole infallible rule of faith.
Part I: The central dilemma
Part II: Sola scriptura and the preacher's authority
Part III: Authority over the Bible
Part II: Sola scriptura and the preacher's authority
Part III: Authority over the Bible
 All Scriptural citations are from the New International Version, a popular Protestant translation used in Evangelical circles.
 Tony Coffey, Once A Catholic (1993), p. 29.
 The date and authorship of 2 Peter are dubious; it may postdate not only the completion of the Gospels but St. Peter's martyrdom (ca. 67).