Mark's gospel has the authority of Peter to back it up:
"'Mark, having become Peter's interpreter, wrote down accurately everything he remembered, though
not in order, of the things either said or done by Christ.'" (Fragments of Papias,
The Apostolic Fathers, Second Edition, J. B. Lightfoot and J. R. Harmer, p. 316).
Luke turns up in Acts, when the voice of the narrator
shifts to 'we'. He makes no claim to have witnessed anything prior to that, but knows those
who did: "Forasmuch as many have taken in hand to set forth in order a declaration of those things
which are most surely believed among us, Even as they delivered them unto us, which from the beginning
were eyewitnesses, and ministers of the word; It seemed good to me also, having had perfect
understanding of all things from the very first, to write unto thee in order, most excellent
Theophilus, That thou mightest know the certainty of those things, wherein thou hast been
instructed." (Luke 1:1-4).
Modern scholars scoff at the idea that any of the gospels
originated amongst people who had any first-hand knowledge of Jesus, or even knew anyone who had.
Their rule of thumb is that anyone who wrote about Jesus must not have known Jesus.
But well-placed observers who lived in that time report it quite differently: "So Matthew
composed the oracles in the Hebrew language and each person interpreted them as best he could."
(Fragments of Papias, from Eusebius, quoted in The Apostolic Fathers, J. B. Lightfoot and J. R.
Harmer, p. 316.)
Justin Martyr likewise, in the mid-second century,
described the gospels as the "memoirs" of the apostles and their followers:
"For in the memoirs which I say were drawn up by His
apostles and those who followed them, [it is recorded] that His
sweat fell down like drops of blood while He was praying and
saying, 'If it be possible, let this cup pass'..."
(Justin Martyr, Dialogue with Trypho, Chapter CIII.)
Tertullian joins the chorus:
"We lay it down as our first position, that the evangelical Testament
has apostles for its authors, to whom was assigned by the Lord Himself this office
of publishing the gospel. Since, however, there are apostolic men also, they are yet
not alone, but appear with apostles and after apostles; because the preaching of disciples
might be open to the suspicion of an affectation of glory, if there did not accompany it
the authority of the masters, which means that of Christ, for it was that which made the
apostles their masters. Of the apostles, therefore, John and Matthew first instill faith
into us; whilst of apostolic men, Luke and Mark renew it afterwards."
(Tertullian, 'Five Books Against Marcion,' Book IV, Chapter 2)
To outweigh Irenaeus', Papias', Justin's and Tertullian's early testimony, where is
the countervailing contemporary testimony? There is none, yet people who
would in most matters endorse the principle of going by the
evidence, in this area prefer to discard all available ancient
evidence and substitute conjecture.
The Unitarians have traversed the whole distance from the
scriptural high ground where they started to the skeptical extremes
of Robert Price and Bart Ehrman, because there are scriptures which
inconvenience them. It would have been better to keep the high
ground, and abandon the errors which had been shown to be such.