The canonical gospels were written at the very most forty to sixty years after Jesus’s death. (p.101 of The Age of Reason)
The four canonical gospels were written much earlier than the so-called Gnostic gospels. The Gospel of Thomas, the best known of the Gnostic documents, is a translation from the Syriac, and scholars have shown that the Syriac traditions in Thomas can be dated to 175 A.D. (sic) at the earliest . . . . (pp.102-103)
The gospels of Matthew, Mark, Luke and John, however, were recognized as authoritative eyewitness accounts almost immediately, and so we have Irenaeus of Lyons in 160 A.D. (sic) declaring that there were four, and only four, gospels. (p.103)
It appears that the very first evidence Keller can find of anyone accepting the canonical gospels as “authoritative eyewitness accounts” was at the very least 90 years after the first gospel was supposedly penned.
Actually Keller’s 160 date for the composition by Irenaeus against heresies is generous in the extreme. We cannot be absolutely sure if Irenaeus was born earlier than 142 c.e., and it was from 161 to 180 that an imperial persecution against Christians was waged. (See Wikipedia Irenaeus.) It was from 180 c.e. that Irenaeus most likely had the time and circumstances to write his many volumes, and 180 c.e. is the date for his writings I usually see referenced.
Justin Martyr around 140 c.e. appears to quote some gospel passages, but he also appears to quote passages from non-canonical gospels, too. So he can hardly have regarded the canonical four as “authoritative” to the exclusion of others.
Ignatius and Polycarp are also highly debatable re how much of their works were late addition or compilations. Keller has no clear evidence of the belief in the canonical gospels as the authoritative “eyewitness accounts” apart from a late second century bishop and apologist for the church headquartered at Rome.
This, in The Age of Reason, is sufficient evidence for him to proclaim:
The gospels of Matthew, Mark, Luke and John, however, were recognized as authoritative eyewitness accounts almost immediately, and so we have Irenaeus of Lyons in 160 A.D. (sic) declaring that there were four, and only four, gospels. (p.103)