Many writings came out of the first century that claimed to be written by an apostle and/or to give accurate accounts of Jesus' life, his teachings, or how to be a Christian disciple. Many of these writings were weeded out by the earliest Christians because they didn't jive with the apostles' actual teaching. The apostles' founded local churches and each church kept their own list of writings to be read in the liturgy; thanks to the consistency of the apostles' teachings it was relatively easy to discard those writings that didn't agree even though churches couldn't communicate with each other easily (and had little incentive to respect another church's authority over their own).
Many writings came from the first century including several Gospel accounts, teaching documents like the Didache, spiritual works like the Shepherd of Hermes, and countless letters by folks like Paul, Peter, James, John, Clement, and others were in wide circulation. Yet today nearly all Christians recognize 27 specific writings as “canon” (meaning “in the list”). Why did Paul's letter to the Romans make it but Clement's didn't? One reason is the consistency of the apostles' teaching.
Before the printing press a story was passed on orally before being written down and this is how it was for Christianity. The apostles went out, preached, and founded churches in all major cities in the known world in a very short amount of time. Thanks to what can only be attributed to efforts of the Holy Spirit, the apostles taught a consistent message even though they could only communicate in rudimentary ways if at all. This oral teaching began a tradition which the churches clung to.
As letters, Gospel accounts, and other writings became available these individual churches (headed by a bishop) took to reading them along with Old Testament passages in the liturgy. However, they wouldn't read just any writing, each bishop kept a list of what he felt were consistent with the apostles' oral preaching.
While not every church in the first few centuries kept the same list, the fact that the apostles taught the same message all across the known world made it possible for the 150+ Christian writings of the first century to be narrowed down and gave a concrete reason to exclude many of the writings.
This is yet another reason Catholics are confident in the canon of Scripture.
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