Luther’s allegiance to the Holy Bible (solo Scriptura) as the ultimate source of authority in Christianity was the most important point of his theology. It is hard for Christians in the 21st century to imagine how the first 350 years of the faith must have been, when the only Bible available was the Old Testament, possibly the Apocrypha, the gospels and the letters of Paul, and some other letters. There was no firm agreement on what constituted the inspired Word of God, and therefore the dangers of heresy were great.
It is little wonder, therefore, that the church came up with a combination of means to judge whether a teaching was orthodox or not. Creeds such as the Apostle’s Creed, which summarized the key points of Christianity, a single bishop in each city who served as the source of authoritative interpretation, and an increasing understanding of which books belonged in the New Testament and which did not, were the means by which the early Church defeated Gnosticism and a host of other heresies that could have destroyed the new faith in its cradle.
The danger, of course, in using such a combination is that believers are tempted to put creeds or authoritative teachers above the Bible. This is exactly what happened. At the Council of Carthage in August 397, church authorities recognized (they did not determine) which books should form the New Testament. These books bore apostolic authority, were consistent with the teachings of the Old Testament, the Gospels, and other Christian teachings, and they were recognized throughout the church as God’s inspired truths.
After the Bible was completed, the reasonable thing to do would have been to recognize that the authority of creeds and bishops is derived from and therefore subordinate to the Word of God. Creeds are useful teaching tools but are essentially mini systematic theologies and therefore necessarily oversimplify or even distort the truths of the Lord. Bishops are usually well educated and faithful and their word can be trusted, but they are still sinful men and liable to errors of omission and commission. Creeds and bishops were still needed because in cultures with few books and little literacy it is important to have such lesser authorities, but God‘s word was the ultimate authority.
What happened, unfortunately, was that the authority of the bishops was magnified, not diminished, until church leaders were presenting as truth teachings that were clearly falsehoods, such as the practice of indulgences. Unbound to the Bible, creeds also proliferated and promulgated misinterpretations of it. Over hundreds of years, these errors gained the authority of tradition and tradition, not the True Word, became the ultimate source of authority. Believers suffered greatly from both of these errors for centuries.
Luther came at a time when mass printing and distribution was possible and literacy was rising fast. Over a millennium after Carthage, he and others rediscovered what should have been apparent all along. The Word of God, not creeds based on it or bishops and other churchmen explaining it, was the real source of truth. Everything believers teach and do must be measured against the only real source of authority, the Bible, and not the other way around.
Tradition still has authority and much of what we do in the modern church, such as worship services at 1100 on Sundays, midweek prayer meetings, and Christmas on December 25th, is based on it. Creeds still have authority, because they are quick and useful ways to teach important Biblical truths to men. However, the Bible is supreme. Luther recognized this, and all of his other great contributions, such as sola fide and sola sacerdos, came from that foundation.