So those who welcomed [Peter's] message were baptized, and that day about three thousand persons were added. They devoted themselves to the apostles’ teaching and fellowship, to the breaking of bread and the prayers (Acts 2:41-2).Before a word of the New Testament was written, the earliest converts to Christianity devoted themselves to the teaching of the apostles as recognizable authorities and sources of knowledge as regards the good news about Jesus of Nazareth. From this verse and many others in the Acts of the Apostles, it is evident that Christianity did not spread randomly and anonymously through word of mouth by unimportant persons. Even if this might have happened, the apostles had a clearly recognized and exercised authority over the teaching of the Christian churches, because they had been Jesus' inner circle and knew his teaching better than anyone else.
It is also clear that, given the occasional nature of the New Testament documents themselves, there is no reason to think that they contain anything like a complete summary of all the apostolic teaching on every important matter. Peter and John and James and Paul and the rest might well have taught much more detailed and sophisticated things than have been written down in the New Testament, though clearly they could not have taught anything contrary to its writings. Paul himself recognizes this distinction between written and oral tradition in this passage:
So then, brothers and sisters, stand firm and hold fast to the traditions that you were taught by us, either by word of mouth or by our letter (2 Thess 2:15).The authority of the apostles translated into the authority of their teachings, regardless of the manner in which these were handed over. An apostolic oral tradition is equally authoritative to a written document, because it traces to an apostle, to one who had seen and learned from Jesus Christ himself. And it is clear, also, that the oral tradition of the apostles preceded and informed the composition and later interpretation of the New Testament.