What is the bottom-most foundation of the Scriptures? From the earliest times, even within the lifetimes of Christ's first Apostles, there have been certain basic criteria for preserving writings and using them as primary sources for the guidance of the Church.
1. The Words of Jesus.
2. The teachings of the Apostles.
3. The story of Jesus' life, as told by those who knew Him, or as faithfully researched from primary sources.
4. Pastoral works written by the Apostles under the inspiration of the Holy Spirit.
5. The history of the first century Church, as recorded by the participants.
The writings that we have designated as "Scriptures" are those which the early Church believed to fit those categories. They are meant to preserve the true "deposit of faith." Thus, the Canon was selected on the consensus that the works chosen were proper and true representatives of the teachings of Christ and his Apostles.
So, while it can be argued that the concept of "sola scriptura" did not exist in the first century of the Church, we must all concede that the principle did indeed exist from the beginning. While yet there were no "Scriptures" to uphold, the Church treasured and followed the teachings and writings that purported to disseminate the teaching of Christ and explain the "Way" that He taught. They preserved those writings and regarded them as highly authoritative, using them to guide the teachings of the presbyters after the first generation of Apostles had passed. Even the first Apostolic Fathers would have kept them as their highest authority, not wanting to distort what their teachers passed down to them in any way.
By remembering the chain of custody of the Scriptures they held, and discerning the clear inspiration of the Holy Spirit, the Church reached a consensus regarding which books and epistles were to be regarded as true and canonical.
With the continuing spread of the Church, and the occasional doctrinal disagreements that emerge, we need to rely on the Canon because it has been established as the consensus of the Church's teachings from its earliest days. The idea that Church authorities might add their own interpretations to the deposit of faith was regarded as spurious at best, or at worst, heretical. Only by the consensus of Ecumenical Councils could authoritative teaching or clarifications of the faith be made incumbent upon all believers. And, further, any such agreements had to be based directly on the Scriptures, as standing upon the authority of the Apostles themselves. Now, in the absence of the Apostles, outside the Canon of Scripture there is no certainty of unquestionable authority.
Later development is not immune to human frailty. The Church can and does bring up ideas inconsistent with true teachings, and these are condemned as heretical. Less obnoxious ideas can still emerge, and the Church can tolerate them for centuries until they appear to be part of tradition. These are clearly accretions. As such, if they have no basis in Scripture, they must not be accredited as established doctrine required for all believers' assent.
The Scriptures are the only certain foundation from which we can, without doubt, discern what Christ and his Apostles intended to teach from generation to generation.
We may regard the authorized doctrine of the Church as true, even with a high degree of certainty, but it is not to be regarded as infallible beyond any possible doubt, especially when it purports to establish its own authority to divide the Church.
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