Does this mean we have to tear up all the pictures of Jesus based on the canonical gospels and start again? No. All kinds of other documents have indeed turned up, not least a whole cache found in Nag Hammadi in Upper Egypt in 1945, some of which give us fascinating glimpses of what people were saying about Jesus at the time of their writing. (The Dead Sea Scrolls, by the way--found not long after the Nag Hammadi documents--say nothing whatever about Jesus or the early Christians, despite many ill-informed assertions to the contrary) But none of them, in fact, is able to trump the gospels we already had.
Is the Gospel of Thomas Legit?
Take the best known, and one of the longest, of the Nag Hammadi documents: a collection of supposed sayings of Jesus known as the Gospel of Thomas. This is the book which, it has often been suggested, could and should be treated as at least equal, and quite possibly superior, to the canonical gospels as a historical source for Jesus himself. The version of Thomas we now have, like most of the Nag Hammadi material, is written in Coptic, a language spoken in Egypt at the time. But it has been demonstrated that Thomas is a translation from Syriac, a language quite like the Aramaic that Jesus must have spoken (though he pretty certainly spoke Greek as well, just as many people in today's world speak English as a second language). But the Syriac traditions that Thomas embodies can be dated, quite reliably, not to the first century at all, but to the second half of the second century. That is over a hundred years after Jesus's own day--in other words, seventy to a hundred years after the time when the four canonical gospels were in widespread use across the early church.
What's more, despite efforts to prove the opposite, the sayings of Jesus as they appear in Thomas show clear indications that they are not as original as the parallel material (where it exists) in the canonical gospels. Sayings have, in many cases, been quietly doctored in Thomas to express a very different viewpoint. For instance, when Jesus says, in Matthew, Mark, and Luke, "Render to Caesar the things that are Caesar's, and to God the things that are God's;" the saying in Thomas has an extra phrase at the end: "and to me the things that are mine." What is going on here? In the worldview represented by Thomas, the word "God" denotes a second-rate kind of deity who made the present wicked world, the world from which Jesus has come to rescue people. Thomas and most of the other Nag Hammadi documents represent a worldview known as "Gnosticism,' in which the present world is a dark, evil place from which we need to be rescued by "gnosis," a special knowledge of hidden truth--a world quite different from the Jewish world of Jesus and the four canonical gospels.
What is more, those four canonical gospels must all have been written by about AD 90 at the very latest. (I am inclined to think they are probably a lot earlier than that, but they cannot be later.) They are known and referred to by Christian writers in the first half of the second century, long before anyone begins to discuss the material we now know from Nag Hammadi. And they incorporate, and are based on, sources both oral and written which go back a lot earlier, sources from the time when not only most of Jesus's followers were still alive and active within the early Christian movement, but when plenty of others--bystanders, opponents, officials--were still around, aware of the new movement as it was growing, and ready to challenge or contradict tales that were gaining currency. Palestine is a small country. In a world without print and electronic media, people were eager to hear and eager to pass on stories about anyone and anything out of the ordinary. The chances are, as John suggests at the end of his gospel, that there was in fact far more material available about Jesus than any one of the gospel writers had space to put down. Source material must have been plentiful. The central features of Jesus's life and work must have been well known. As one of the early preachers says, these things were not done in a corner.