How can we really know for sure that the Gnostic gospels- The gospel of Thomas, the gospel of Philip, the gospel of Mary, the gospel of Judas, etc. were actually not written by the disciples of Jesus Christ?
I suppose we cannot absolutely “prove” in the mathematical sense of proof that Thomas did not write Thomas or Philip did not write Philip, etc., but the evidence is extremely strong that they did not. In fact, the evidence is so strong that it borders on irrational to conclude otherwise. Let me explain.
First of all, among actual scholars, even the hardened critics and skeptics of Christianity will all agree that the four gospels were written in the first century. They try to push these letters as late as possible, but even they cannot push Matthew Mark or Luke past AD 80, nor can they push John past AD 100. The actual dates are more like this: Mark: late 50s or early 60s Matthew: 60s Luke: about AD 63 or 64 John: 80s or early 90s AD. The fact is that the four gospels were already being quoted by church leaders in the turn of the second century. Clement of Rome, writing around AD 96 quotes from at least two of the gospels. The Didache, of unknown authorship, but written most likely in the very late first century, also quotes from the canonical gospels.
The situation with the supposed Gnostic “gospels” is completely different. There is absolutely zero evidence any of these were written in the first century. There have been a couple of fringe scholars who have claimed Thomas was written in the late first century, but the evidence simply does not support this conclusion. We know that Thomas and Judas were written for sure by some time in the second century because Irenaeus quotes them in his letter “Against Heresies” in about AD 190. Let us concede that most likely the four supposed gospels you mention were written in the second century, which is what the evidence points toward. This is the time when Gnosticism was rampant in some of the churches. If this is so, then we can be well assured that the Thomas, Philip, Mary of Judas of the Bible did not write these books. Certainly Judas did not write the falsely-named Gospel of Judas!!!! These books are fraudulently attributed to New Testament characters. The word “fraudulent” may be a bit too harsh, given that it was accepted practice in the Jewish and Greek world to wrote a pseudonymous (ie falsely attributed) book under the name of a person who did not actually write the book. This was not necessarily considered unethical, but was a literary device. Examples are 1 and 2 Enoch, which were read by the Jews, but everyone know that Enoch did not write these books. So, we can assume that people reading the “Gospel of Thomas” in the second or third century obviously knew that Thomas did not write this document. Therefore my use of the word fraudulent is probably a bit too strong.
Nevertheless, it should be noted that even the Gnostics never included the Gospel of Thomas or any other of their Gnostic writings in their canon of Scripture. In other words, even the Gnostics did not have the nerve to claim that their falsely-attributed writings were inspired in the same sense that the four gospels were inspired.
My conclusion is that there is absolutely no way that these supposed other “Gospels” were actually written by people who met Jesus personally or that they had apostolic authority.