Here is the historic, formal, official definition. From 1577:
"The Scriptures are and should remain the sole rule in the norming of all doctrine among us"
(Lutheran Formula of Concord, Solid Declaration, Rule and Norm, 9).
The Rule of Scripture is the practice of embracing Scripture as the rule ("straight edge") - canon ("measuring stick") - norma normans (the norm that norms) as it is called in epistemology, as we examine and evaluate the positions (especially doctrines) among us.
What it IS:
1. An embrace of accountability for the doctrines among us (especially those in dispute).
2. An embrace of norming (the process of examining positions for truth, correctness, validity).
3. An embrace of Scripture as the best, most sound rule/canon/norma normans for US to USE for THIS process.
What it is NOT:
1. A teaching that all revelation or truth is found in Scripture. It's not a teaching at all, it is the PRACTICE of using Scripture as the rule in the norming of doctrines. Scripture itself says that "the heavens declare the glory of God" but our visual reception of the stars is not used as the norma normans for the evaluation of doctrines among us in the practice of Sola Scriptura.
2. A teaching that Scripture is "finished." It's not a teaching at all. While probably all that practice Sola Scripture agree with all others that God seems to have inscribed His last book around 100 AD and doens't seem to be adding any more books, the Rule of Scripture was just as "valid" in 1400 BC when Scripture consisted of just two stone tablets as it is today - only the corpus of Scripture is larger, that has no impact on the practice of embracing it as the rule/canon/norma normans in our evaluation of doctrines among us. The Rule of Scripture embraces the Scripture that is.
3. Hermeneutics. The Rule of Scripture has to do with WHAT is the most sound rule/canon/norma normans for the evaluation of the doctrines among us, it is not a hermeneutical principle. Obviously that Scripture needs to be interpreted, but that's a different subject or another day and thread. The Rule of Scripture has to do with norming, not interpreting.
4. Arbitration. Obviously, some process of determining whether the doctrine under review "measures up" (arbitration) to the "measuring stick" (the canon). This is also beyond the scope here, the Rule of Scripture is the embrace of Scripture AS that canon, it does not address the issue of HOW it is best determined if a position "measures up" to that canon.
An illustration:
Let's say Dave and Fred are neighbors. They decided that they will hire a contractor to build a brick wall on their property line, six feet tall. Dave and Fred hire Bob the Builder. He agrees to build the wall on the property line - six feet tall.
Bob is now done. He claims the wall is six feet tall. Does it matter? If it doesn't, if his work and claim are entirely irrelevabnt - then, nope - truth doesn't matter. And can just ignore what he said and did. OR we can consider that of the nearly 7 billion people in the world, there is ONE who is incapable of being wrong about measurements - and that ONE is Bob the Builder, claims ONE - Bob the Builder. IF Bob the Builder alone is right about what he alone claims about he alone here, it's pretty much a waste of time to wonder if what he said about this is true or not. But, IF truth matters and IF Bob the Builder will permit accountability (perhaps because he is confident the wall IS six feet tall), then we have the issue of accountability: Is the wall what we desire and what Bob the Builder claims it is?
If so, we just embraced norming. Norming is the process of determining correctness of the positions among us. For example, Bob claiming the wall is 6 feet tall. Is that correct? Addressing that question is norming.
Norming typically involves a norm: WHAT will serve as the rule (straight edge) or canon (measuring stick) - WHAT will be embraced by all parties involved in the normative process that is the reliable standard, the plumbline. Perhaps in the case of Fred and Dave, they embrace a standard Sears Measuring Tape. They both have one, Bob does too. Dave, Fred and Bob consider their carpenter's Sears Measuring Tape as reliable for this purpose, it's OBJECTIVE (all 3 men can read the numbers), it's UNALTERABLE (none of the 3 can change what the tape says) and it's OUTSIDE and ABOVE and BEYOND all 3 parties. Using that could be called "The Rule of the Measuring Tape." The Sears Measuring Tape would be the "canon" (the word means 'measuring stick') for this normative process.
Why Scripture?
In epistemology (regardless of discipline), the most sound norma normans is usually regarded as the most objective, most knowable by all and alterable by none, the most universally embraced by all parties as reliable for this purpose. My degree is in physics. Our norma normans is math and repeatable, objective, laborative evidence. Me saying, "what I think is the norm for what I think" will be instantly disregarded as evidential since it's both moot and circular. I would need to evidence and substantiate my view with a norm fully OUTSIDE and ABOVE and BEYOND me - something objective and knowable. This is what The Handbook of the Catholic Faith proclaims (page 136), "The Bible is the very words of God and no greater assurance of credence can be given. The Bible was inspired by God. Exactly what does that mean? It means that God Himself is the Author of the Bible. God inspired the penmen to write as He wished.... the authority of the Bible flows directly from the Author of the Bible who is God; it is authoritative because the Author is." Those that accept the Rule of Scripture tend to agree. It's embrace as the most sound Rule flows from our common embrace of Scripture as the inscriptured words of God for God is the ultimate authority.
The embrace of Scripture as the written words of God is among the most historic, ecumenical, universal embraces in all of Christianity. We see this as reliable, dependable, authoritative - it as a very, very, broad and deep embrace as such - typically among all parties involved in the evaluation. (See the illustration above).
It is knowable by all and alterable by none. We can all see the very words of Romans 3:25 for example, they are black letters on a white page - knowable! And they are unalterable. I can't change what is on the page in Romans 3:25, nor can any other; what is is.
It is regarded as authoritative and reliable. It is knowable by all and alterable by none. Those that reject the Rule of Scripture in norming ( the RCC and LDS, for example ) have no better alternative (something more inspired, more inerrant, more ecumenically/historically embraced by all parties, more objectively knowable, more unalterable), they have no alternative that is clearly more sound for this purpose among us.
To simply embrace the teachings of self (sometimes denominational "tradition" or "confession") as the rule/canon is simply self looking in the mirror at self - self almost always reveals self. In communist Cuba, Castro agrees with Castro - it has nothing whatsoever to do with whether Castro is correct. We need a Rule outside, beyond, above self.
Why does the RCC so passionately reject this practice?
Those that reject the Rule of Scripture in norming tend to do so not because they reject Scripture or have an alternative that is MORE inerrant, MORE the inscripturated words of God, MORE reliable, MORE objectively knowable, MORE unalterable, MORE ecumenically embraced as authoriative, MORE above and beyond and outside all disputing parties. Rather the rejection is because the protestors rejects accountability (and thus norming and any norm in such) in the sole, singular, exclusive, particular, unique, individual case of it itself alone, uniquely, individually. From The Handbook of the Catholic Faith (page 151), "When the Catholic is asked for the substantiation for his belief, the correct answer is: From the teaching authority. This authority consists of the bishops of The Catholic Church in connection with the Pope in Rome. The faithful are thus freed from the typically Protestant question of 'is it true' and instead rests in quiet confidence that whatever the Catholic Church teaches is the teaching of Jesus Himself since Jesus said, 'whoever hears you hears me'." The Catholic Church itself says in the Catechism of itself (#87): Mindful of Christ's words to his apostles: “He who hears you, hears me”, The faithful receive with docility the teachings and directives that their pastors give them in different forms." IF self declares that self is unaccountable and that self is exempt from the issue of truthfulness, then the entire issue of norming (and the embraced norma normans in such) becomes entirely irrelevant (for itself). The issue has been changed from truth to power (claimed by itself for itself, exclusively).
The RCC's "fathers" on Sola Scriptura:
"We are not entitled to such license, I mean that of affirming what we please; we make the Holy Scriptures the rule and the measure of every tenet; we necessarily fix our eyes upon that, and approve that alone." St. Gregory of Nyssa (On the Soul and the Resurrection NPNF II, V:439) Note: Gregory of Nyssa died in 394, quite a few years BEFORE Luther or Calvin were born. Compare to the definition of the practice.
Here are just a few other cases....
"Regarding the things I say, I should supply even the proofs, so I will not seem to rely on my own opinions, but rather, prove them with Scripture, so that the matter will remain certain and steadfast." St. John Chrysostom (Homily 8 On Repentance and the Church, p. 118, vol. 96 TFOTC)
"Let the inspired Scriptures then be our umpire, and the vote of truth will be given to those whose dogmas are found to agree with the Divine words." St. Gregory of Nyssa (On the Holy Trinity, NPNF, p. 327).
"What is the mark of a faithful soul? To be in these dispositions of full acceptance on the authority of the words of Scripture, not venturing to reject anything nor making additions. For, if ‘all that is not of faith is sin' as the Apostle says, and ‘faith cometh by hearing and hearing by the Word of God,' everything outside Holy Scripture, not being of faith, is sin." Basil the Great (The Morals, p. 204, vol 9 TFOTC).
"We are not content simply because this is the tradition of the Fathers. What is important is that the Fathers followed the the Scripture." St. Basil the Great (On the Holy Spirit, Chapter 7, par. 16)
For concerning the divine and holy mysteries of the Faith, not even a casual statement must be delivered without the Holy Scriptures; nor must we be drawn aside by mere plausibility and artifices of speech. Even to me, who tell you these things, give not absolute credence, unless you receive the proof of the things which I announce from the Divine Scriptures. For this salvation which we believe depends not on ingenious reasoning, but on demonstration of the Holy Scriptures. St. Cyril of Jerusalem (Catechetical Lectures, IV:17, in NPNF, Volume VII, p. 23.)
Neither dare one agree with catholic bishops if by chance they err in anything, but the result that their opinion is against the canonical Scriptures of God. St. Augustine (De unitate ecclesiae, chp. 10)
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