Dan Brown and The Da Vinci Code: The Gospel of Thomas
The Gospel of Thomas is one of the most important documents for New Testament specialists, outside the Bible. It is a collection of Jesus's sayings, some of which are very similar to the sayings that are found in the Gospels in the New Testament.
Some specialists have argued that the Gospel of Thomas is earlier than any of the existing Gospels in the Bible. However, there are three good reasons for believing this is not the case:
(1) Manuscript evidence
The manuscript evidence for the Gospel of Thomas is later than the Gospels that are in the Bible:
The Coptic text of the Gospel of Thomas, from the Nag Hammadi library, comes from about 350 AD. The earliest fragmentary Greek text comes from around 200 AD. This is important, because the Gospels that are in our Bible come from the last quarter of the first century AD.
The belief that the Gospel of Thomas is earlier than the Gospels in the Bible is not based on the evidence from manuscripts, but on particular theories about how the Gospels came in to being.
(2) Thomas quotes from documents that are in the Bible
The Gospel of Thomas quotes from, or alludes to, many of Paul's letters in the Bible - to Romans, 1 and 2 Corinthians, Galatians, Ephesians, Colossians, 1 Thessalonians and 1 Timothy. It also alludes to Hebrews, 1 John, and Revelation. The writer of the Gospel of Thomas was familiar with all four of the Gospels in the Bible. These books must have been written, and circulated, before the Gospel of Thomas was written.
(3) Marcion does not quote from Thomas
Marcion was a sort of fore-runner of the Gnostics, who flourished around 144 AD. The teaching of the Gospel of Thomas fits in well with his ideas, so we might expect him to quote from it. However, he does not mention it at all or allude to it. The most likely reason is that it did not exist in his day. (This is an argument from silence, so it is not definite proof. However, it does support the first two points.)