Whether all God made, all man made, or a combination, doctrine was certainly transmitted by fallible human beings, and human beings who interpreted things that weren't perfectly clear to them, so error is possible -- but by and large, I think we have a pretty good handle on proper Christian doctrine even today.
It's nice to have a written record of Christ's teachings, but surely there were many more teachings that didn't get written down (or did, but they are now lost to us). There had to be a ton of that, right? The very last verse of John's gospel is pertinent here. So, the apostles' oral transmission to others, and the transmittees' further transmission, is a source of doctrine too.
If Eusebius is to be believed in Book III, ch. 39 of his Church History,
CHURCH FATHERS: Church History, Book III (Eusebius), Papias actually preferred the oral to the written, which is kinda strange:
2. But
Papias himself in the preface to his discourses by no means declares that he was himself a hearer and eye-
witness of the
holy apostles, but he shows by the words which he uses that he received the doctrines of the
faith from those who were their friends.
3. He says: But I shall not hesitate also to put down for you along with my interpretations whatsoever things I have at any time learned carefully from the elders and carefully remembered, guaranteeing their
truth. For I did not, like the multitude, take pleasure in those that speak much, but in those that teach the
truth; not in those that relate strange commandments, but in those that deliver the commandments given by the
Lord to
faith, and springing from the
truth itself.
4. If, then, any one came, who had been a follower of the elders, I questioned him in regard to the words of the elders — what
Andrew or what Peter said, or what was said by Philip, or by
Thomas, or by James, or by John, or by Matthew, or by any other of the
disciples of the
Lord, and what things Aristion and the
presbyter John, the
disciples of the
Lord, say.
For I did not think that what was to be gotten from the books would profit me as much as what came from the living and abiding voice.