So considering that the oldest copies of the gospels are dated to around 400 AD (I'm thinking of the Codex Sinaiticus), how do scholars go about estimating the date of composition of the gospels? I mean I'm sure there are some textual clues; I mean I assume the usage of Koine would probably change somewhat over 300+ years but are there other clues that scholars use to estimate the general time of composition?
While the Codex Sinaiticus dates from the 4th century, other manuscript fragments date much earlier. The Greek unical codices provide important clues to the development of the Canon, but are less important as evidence of the date of composition. For instance, this is a fragment of the Gospel of John:
Dating the papyrus scrap is difficult, but based on the style of the script used, it's probably between 117 CE and 138 CE. (Slightly larger ranges are more-likely to include the actual date, but also begin to lose accuracy. Circa 125 CE is the standard single-point estimate.) Because the codex P52 came from was written in Alexandria and the gospel originated elsewhere, the gospel of John must go back to the 1st century.
The Synoptic Problem
If we can date John to the 1st century, we can have some confidence that the other gospels are at least as old. For one thing, there's evidence that the author of John was aware of the synoptic gospels. In addition, many scholars believe that Luke and Matthew had Mark available when they wrote their biographies of Jesus. They additionally might have had access to an even earlier source. For our purposes, the exact solution to the synoptic problem doesn't matter; what does matter is that the gospels where not written at the same time.
If we fix P52 at 125 CE and string the dependencies together, we get a timeline like this:
Jesus <- Q <- Mark <- Luke/Matthew <- John <- P52 30 or 33 <- t1 <- t2 <- t3 <- t4 <- c. 125 CE
So in the ~92 years between Jesus' life and P52 we need to fit in this sequence of development. There's certainly plenty of leeway here. Mark (
t2) has been dated as late as 80 CE, which would bunch all the gospels to the end of the 1st and beginning of the 2nd century. On the other hand, the dates could be clustered nearer to the middle of the 1st century without straining the timeline or credibility.
Finally, there is the critical question of how the gospels relate to the destruction of the Temple in Jerusalem. Internal evidence strongly suggests that Mark was written either while the Roman legions were moving toward the siege of Jerusalem or shortly afterward. It could very well be that the Great Revolt was a motivation for the composition of Mark as the Jerusalem church and its eyewitness tradition would have been in peril. If so, all gospels dependent on Mark (i.e., the other Synoptics) must originate from the decades following 70 CE.
Since Paul's undisputed letters can all be dated from 50 to 60 CE, the picture that emerges is a nearly continual stream of Christian writing from two decades after the crucifixion to today. Much of the New Testament seems to have been written in response to particular problems the nascent religion actually faced.
Unfortunately, Koine Greek, the trade language that the New Testament was written in, seems not to be useful for dating the text of the Gospels. Some have speculated that Matthew was originally an Aramaic text, but since the earliest manuscripts of Matthew are in Greek, that remains a speculation. If it could be shown that Matthew was originally written in the language Jesus likely spoke, we could date that gospel much earlier. As it is, the linguistic evidence is minimally useful, at best, when it comes to dating the New Testament.
Dating of ancient texts is as much an art as a science. Dating philosophy both derives from and informs hermeneutical assumptions, so an accurate timeline that is universally accepted seems impossible. (Though more manuscript discoveries might bring more certainty.) Therefore, I offer this as my personal best estimate:
Jesus <- Q <- Mark <- Luke/Matthew <- John <- P52 30 or 33 <- 40? <- 70? <- 80? <- 90? <- c. 125 CE