In my answer to Brian Holtz’s challenge, I make the following statement:
The Gospel of Luke begins in the following manner:
Since many have undertaken to set in order a narrative concerning those matters which have been fulfilled among us, even as those who from the beginning were eyewitnesses and ministers of the word delivered them to us, it seemed good to me also, having traced the course of all things accurately from the first, to write to you in order, most excellent Theophilus; that you might know the certainty concerning the things in which you were instructed.
John answered, "Master, we saw someone casting out demons in your name, and we forbade him, because he doesn't follow with us." Jesus said to him, "Don't forbid him, for he who is not against us is for us."
Apparently Luke was not worried that there are competing Christian voices because neither is Jesus Christ worried. That has been the natural state of affairs in Christianity from the beginning. But neither do Luke and Christ doubt that there is an authoritative voice in the cacophony, and that such a voice is readily distinguished. It is distinguished as the voice of authoritative tradition [Luke 24:45-49]:
Then he opened their minds, that they might understand the Scriptures. He said to them, "Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in his name to all the nations, beginning at
The situation is continued in the Pauline epistles. A regular concern in the epistles is Paul’s struggles to keep the churches he founded from straying from the true Faith. Time and again he admonishes his flock not to be tempted to follow other voices. He gives reasons and defenses of the Faith for sure, but the fundamental reason he gives that his Gospel should be followed is the authority with which he taught it.
What we know of early Christianity indicates that it was a lot like late-Roman Christianity, medieval Christianity, early-modern Christianity, and contemporary Christianity. A lot of people running around claiming to have the true interpretation of the faith, with one steady, authoritative voice consistently proclaiming the same Gospel across time. This is why I don't get worked up when the newspapers breathlessly announce the discovery of a new Gospel of Thomas, Gospel of Judas or the remains of some bizarre quasi-Christian first century sect, or when scholars announce a radical new interpretation of the Bible that puts paid to orthodox faith. Of course there were all varieties of heresies proclaimed right from the beginning; and of course the New Testament documents can be forced into supporting non-Christian doctrines. It was ever thus. And it gives no reason for supposing that the Church's witness to the truth has not been consistent from the beginning.