Simply stated, the Protestant doctrine of sola scriptura ("Scripture alone") holds that every point of Christian theology -- everything pertaining to "faith and practice" -- must be verifiable from the Bible alone. This is expressed by the old Protestant slogan Quod non est biblicum, non est theologicum ("What is not biblical is not theological").
An essential part of this doctrine, as it has been historically articulated by Protestants, is that theology must be done without allowing Tradition or a Magisterium (teaching authority) to have binding authority in how Scripture is to be interpreted. If Tradition or a Magisterium could bind the conscience of the believer as to what he was to believe, then the believer would not be looking to Scripture alone as his authority.
A necessarily corollary of the doctrine of sola scriptura is an absolute right of private judgment in the interpretation of the Scriptures. Each individual has the final prerogative to decide for himself what the correct interpretation of a given passage of Scripture is, regardless of what anyone or everyone else says. Thus, according to sola scriptura, any role that Tradition, a Magisterium, Bible commentaries, or anything else may play in theology should do no more than suggest interpretations and evidence to the believer as he makes his decision. Each individual is put in the position of being his own theologian.
Of course, the average Christian does not exercise this role in any consistent way. (There are many godly grannies who are very devout in their faith in Jesus, but who are in no way inclined to become theologians.) Not only is the average Christian disinclined to fulfill the role of theologian, if he tries to do so and arrives at conclusions different than those of his church's leadership, he will quickly discover that his right to private judgment amounts to a right to shut up or leave the congregation.
Protestant pastors from the time of Luther and Calvin have realized that, although they must preach the doctrine of private judgment to ensure their own right to interpret Scripture, they must prohibit the exercise of this right to others, lest their group be torn apart by strife. It is the failure to prohibit the right of private judgment that has resulted in the over twenty thousand Christian Protestant denominations listed in the Oxford University Press World Christian Encyclopedia.
These churches are divided over questions like: What kind of faith saves? Is baptism necessary? Is it needed? Is it for infants? Can one lose salvation? How? Can it be gotten back? How? Is the Real Presence true? Are spiritual gifts like tongues and healing for today? For everyone? What about predestination? What about free will? What about church government?
The disintegration of Protestantism into so many competing factions, each teaching different doctrines on key theological issues, is itself an important indicator of the practical failure of the doctrine of sola scriptura. But even beyond this, there is a set of practical presuppositions that sola scriptura makes, every one of which provides not just an argument against the doctrine but a fatal blow to it.
If God had intended the average Christian to use sola scriptura as an operating principle, then it would have to be something the average Christian could implement. If it turns out that the average Christian in world history could not have implemented it, then it must not be God's plan.
In fact, the average Christian in each age of Church history would have to be able to use sola scriptura, since Jesus promised that his Church would never pass out of existence but would be present throughout the rest of history (Matt. 16:18, 28:20).
If each Christian is to make a thorough study of the Scriptures and decide for himself what they mean -- even taking into consideration the interpretations of others -- then it follows that he must have a copy of the Scriptures to use in making his thorough study. A non-thorough study is a dangerous thing, as any Protestant apologist warning one against cults and their Bible study tactics will tell you. The universal application of sola scriptura therefore presupposes the printing press and the mass manufacturing of books and of the Bible in particular.
Without mankind's ability to mass-produce copies of the Scriptures for individual Christians to interpret, the doctrine of sola scriptura could not have functioned. Christians in the days before the printing press had only limited access to the texts via the Scripture readings at Mass and the costly, hand-made copies of the Bible kept on public display in parishes. But these did not allow the average believer the lengthy, detailed access to Scripture that he needed to serve as his own theologian.
It is often noted, even by Protestant historians, that the Reformation could not have blossomed as it did in the early 1500s if the printing press had not been invented in the mid-1400s. It took a couple of generations for the idea of printing to make its mark on the European imagination. It was in this heady atmosphere, the first time in human history when dozens of ancient works were being mass produced and sold, that people suddenly thought, "Hey! We could give copies of the Bible to everyone! Everyone could read the Scriptures for themselves!"
Moreover, the printing press not only allowed the early Protestants to mass-produce the Bible, it meant they had a means to disseminate their own works about what the Bible meant. In the minds of those who wished to oppose historic Christian theology, this prospect led very quickly to the idea sola scriptura, as it provided a justification for their own desire to depart from orthodoxy.
Besides the printing press, sola scriptura also presupposes a universal distribution system of books and of the Bible in particular. It is no good for enough copies of the Bible to exist if they can't be gotten into the hands of the average believer. There must be a distribution network capable of delivering affordable copies of the Bible to the average Christian.
Throughout the great majority of Christian history, the universal distribution of books would have been impossible even in what is now the developed world. During most of Church history, the "developed world" was undeveloped. The political systems, economies, and travel infrastructure that make the mass distribution of Bibles possible today simply did not exist for three-quarters or more of Church history. And even now we cannot get enough Bibles into many lands due to economic and political restraints, as the fund-raising appeals of Bible societies and their stories of Bible-smuggling show us.
Just as there was no way to get the books to the peasants, there was no way the peasants could have afforded them in the first place. This would have required a cash-based economy, and there just wasn't enough cash in circulation.
A barter-based economy would never work with mass-produced and distributed goods. Imagine a medieval peasant giving a printer ten thousand chickens for one thousand Bibles. Even if the printer agreed to the deal, the peasant would have the daunting task of keeping the chickens alive and fed and transported from the time he relinquished them to the time the printer received them.
Sola scriptura also presupposes universal literacy. If the average Christian is going to read the Scriptures and decide for himself what they mean, then obviously he must be able to read. Having someone read the Bible to him is not sufficient. Not only would the reader be able to do it only occasionally (since there would be many illiterates to read to), the person who wishes to be his own theologian needs to go over the passage multiple times, looking at its exact wording and grammatical structure. He needs to be able to quickly flip to other passages bearing on the topic, and he needs to be able to record his insights so he doesn't forget them and so he can keep the evidence straight in his mind.
Anyone who is to make a study Scripture and decide what it teaches must possess adequate scholarly reference works, or else he must be able to read the texts in the original languages. For example, does the Greek word for "baptize" mean "immerse" or does it have a broader meaning? Does the biblical term for "justify" mean to make righteous in only a legal sense or sometimes in a broader one?
He must also have commentaries to suggest to him possible alternate interpretations. No one person is going to be able to think of every possible interpretation of every Scripture passage that is relevant to every major Christian doctrine.
No Protestant pastor (or at least no pastors who are not extreme anti-intellectuals) would dream of formulating his views without such support materials, and he thus cannot expect the average Christian to do so either. Indeed, the average Christian is going to need such support materials even more than a trained pastor.
If the average Christian is to do a thorough study of the Bible for himself, he must have adequate leisure time. If he is working in the fields or a home (or, later, in the factory) for ten, twelve, fifteen, or eighteen hours a day, he doesn't have time to do this, especially in addition to the care and raising of his family and his own need to eat, sleep, and recreate.
Not even a Sunday rest will provide him with the adequate time; nobody becomes adept in the Scripture by reading the Bible only on Sundays. Protestants stress this to their own members when encouraging daily Bible reading. Thus sola scriptura presupposes the universal possession of adequate leisure time in which to make a thorough study the Bible for oneself.
If the average Christian is going to evaluate competing interpretations for himself then he must have a significant amount of critical thinking skills in evaluating arguments. He must be able to recognize what is a good argument and what is not, what is a fallacy and what is not, what counts as evidence and what does not.
As anyone who has ever tried to teach basic logic to college students or read and grade their persuasive essays can tell you, that level of critical thinking does not exist in the average, literate, modern college senior, much less the average, illiterate, medieval peasant. This is especially true when it comes to the abstract concepts and truth claims involved in philosophy and theology. Thus sola scriptura also presupposes a high level of universal education in critical thinking skills-in fact, it presupposes a level of critical thinking skill that does not exist in the average college student today.
In summary, sola scriptura presupposes (1) the existence of the printing press, (2) the universal distribution of Bibles, (3) a cash-based economy, (4) universal literacy, (5) the universal possession of scholarly support materials, (6) the universal possession of adequate time for study, and (7) a universal education in a high level of critical thinking skills.
Needless to say, this group of conditions was not met in the crucial early centuries of the Church, was not met through the main course of Church history, and is not met even today. The non-existence of the printing press alone means sola scriptura was totally unthinkable for almost three-quarters of Christian history.
It is thus hard to think of sola scriptura as anything but the theory spawned by a group of Renaissance-era dilettantes-people who had an interest in being their own theologians, who had a classical education in critical thinking skills, who had plenty of leisure time for study, who had plenty of scholarly support materials, who had good reading skills, who had access to Bible-sellers, and most importantly, who had printed Bibles.
The average Christian today -- even the average Christian in the developed world -- does not fit that profile. Much less did the average Christian in the early centuries. What this means, since God does not ask a person to do what they are incapable of doing, is that God does not expect the average Christian of world history to use sola scriptura. He expects the average Christian to obtain and maintain his knowledge of theology in some other way.
But if God expects the average Christian to obtain and maintain the Christian faith without using sola scriptura, then sola scriptura is not God's plan.