The Gospel of Thomas is a Coptic manuscript discovered in 1945 at Nag Hammadi in Egypt. This manuscript contains 114 sayings attributed to Jesus. Some of these sayings resemble sayings found in the Gospels of Matthew, Mark, Luke, and John. Other sayings were unknown until their discovery or even run counter to what is written in the four Gospels.
One December day in 1945, far up the Nile Valley, two Egyptian peasants were looking for a local variety of crumbly nitrate rock used as fertilizer. They came across a large jar, about a meter tall, hidden by a boulder. Inside they found a collection of ancient leather-bound books or codices. The spot where the books were found is within a few miles of the site of an early monastery, established by the founder of Christian "cenobitic" monasticism in Egypt, Pachomius. Nag Hammadi, a nearby village, has given this remarkable collection its name.
The Nag Hammadi Library consists of fifty-two texts or "tractates" written in Coptic on papyrus and gathered in thirteen volumes, twelve of which have separate leather bindings. Forty of the texts had previously been unknown to modern scholars. Most of the writings are of a Gnostic character. Scraps of paper found in the binding of eight codices bear dates indicating that the books were made in the mid-fourth century, and at least one of these clearly appears to have come from a monastery. Efforts to date the books more precisely continue. In general, it can be said the collection dates from about the middle of the fourth century. The Coptic texts could be many years earlier, and the originals (probably written in Greek or Aramaic) from which the Coptic translations were made could have been still earlier.
To understand how we got the Bible as we know it, please see the following two articles: What is the canon of Scripture? and How was the Canon determined?
Should the Gospel of Thomas be in the Canon?
The early church councils followed something similar to the following principles to determine whether a New Testament book was truly inspired by the Holy Spirit: 1) Was the author an apostle or have a close connection with an apostle? 2) Was the book being accepted by the Body of Christ at large? 3) Did the book contain consistency of doctrine and orthodox teaching? 4) Did the book bear evidence of high moral and spiritual values that would reflect a work of the Holy Spirit?
The Gospel of Thomas fails all of these tests. The Gospel of Thomas was not written by Jesus’ disciple Thomas. The early Christian leaders universally recognized the Gospel of Thomas as a forgery. The Gospel of Thomas was rejected by the vast majority of early Christians. The Gospel of Thomas contains many teachings that are in contradiction to the biblical Gospels and the rest of the New Testament. The Gospel of Thomas does not bear the marks of a work of inspiration of the Holy Spirit.
Are there any other arguments that preclude the Gospel of Thomas from being included in the Bible? If we examine the 114 sayings in this writing, then we find some that are similar to existing sayings, some that are slightly different, but the majority cannot be found anywhere in the entirety of Scripture itself. Scripture must always confirm itself, and the majority of sayings in the Gospel of Thomas cannot be confirmed anywhere else in Scripture.
One argument for precluding the Gospel of Thomas from the Bible is found in the overt "secretness" attributed to these 114 sayings by the work itself. Nowhere in Scripture is God’s Word given “in secret" but is given for all to read and understand. The Gospel of Thomas very clearly tries to maintain an air of secrecy in its words.
The Gospel of Thomas is a Gnostic gospel, espousing a Gnostic viewpoint of Christianity. The Gospel of Thomas is simply a heretical forgery, much the same as the Gospel of Judas, the Gospel of Mary, and the Gospel of Philip. Perhaps the disciple Thomas' nickname of "doubting Thomas" is appropriate here. We should all be doubting the Gospel of Thomas!