Gospel of Thomas - Chapter 054
- (52) His disciples said to him, "Twenty-four prophets spoke in Israel, and all of them spoke in you."
He said to them, "You have omitted the one living in your presence and have spoken (only) of the dead."
Arrgh! I was on a roll. Sorry.
Some keys to consider,
- Israel started with Jacob.
- Prophets in Judah probably don't count as prophets in Israel, since the nation was split.
- Many prophets were mentioned, but we don't know what they said, therefore they didn't 'speak'.
- There are a couple possibilities for the one who is alive. It could be John, who is an Old Testament prophet, was alive, and by that time Judah was known as Israel again. The other would be Jesus.
It is a reference to the Jewish canon.
- Bible: the written Torah. The Bible comprises twenty-four books, divided into three sections: (1) the Torah ("teaching"), comprising the five books of Moses; (2) the eight books of the Prophets (Nevi'im, the first and second books of Samuel and Kings are considered one book, as are the twelve "minor" prophets); (3) the eleven books of the Writings (Ketuvim, the books of Ezra and Nehemiah are considered one book, as are the two books of Chronicles). The Bible is therefore known in Hebrew as the Tanach, the abbreviation formed by the first letters of the names of these three sections. -- 
The riddle refers to the books of the Tenach as prophets and it's solution is significant since Thomas is an early source, it adds weight to the idea that the canonization of the Jewish Bible took place early, and endorses the canon as being accepted by Jesus, and therefore pushes the date of canonization to a very early date.
- Rabbinic Judaism (Hebrew: יהדות רבנית) recognizes the twenty-four books of the Masoretic Text, commonly called the Tanakh (Hebrew: תַּנַ"ךְ) or Hebrew Bible. Evidence suggests that the process of canonization occurred between 200 BC and 200 AD, and a popular position is that the Torah was canonized c. 400 BC, the Prophets c. 200 BC, and the Writings c. 100 AD perhaps at a hypothetical Council of Jamnia—however, this position is increasingly criticized by modern scholars. 
This is a good time to mention the nature of riddle; A riddle cannot be solved using the contents of the riddle itself. Other information must be available. This is why riddles are used to hide things. The thing must already be known. Christ can effectively be hidden in the OT by using riddles because no one knew of him. The New Testament is a record of his life and death, and once that is known, the riddles of the OT can be understood.
Since my Bible is divided differently than the Tenach, there is no way I should be able to solve this riddle without the additional information on how the Jewish Bible is divided.
In this case the living prophet would have been Jesus himself. The riddle says that Jesus was the voice behind all the dead prophets.