Proving the existence of Jesus as a historical person
The Jesus Seminar is a large group of scholars dedicated to finding the authentic sayings of the historical Jesus. They examined the transmission of sayings from Jesus to the writing of the 5 gospels (Thomas included).
Jesus taught his followers orally. The oral memory best retains sayings and anecdotes that are shown provocative, memorable, and oft-repeated. The most frequently recorded words of Jesus in the surviving gospels take the form of aphorisms and parables. The earliest layer of the gospel tradition is made up of single aphorisms and parables that circulated by word of mouth prior to the written gospels.
With these facts considered, the Seminar formulated key characteristics to authentic the sayings of Jesus. (Note: The seminar only considers 18% of the sayings in the 5 gospels to be authentic to the historical Jesus)
-Jesus' characteristic talk was distinctive. It can usually be distinguished from common lore. Otherwise it is futile to search for the authentic words of Jesus.
-Jesus' sayings and parables cut against the social and religious grain.
-Jesus' sayings and parables surprise and shock: they characteristically call for a reversal of roles or frustrate ordinary, everyday expectations.
-Jesus' sayings and parables are often characterized by exaggeration, humor and paradox.
-Jesus' images are concrete and vivid, his sayings and parables customarily metaphorical and without explicit application.
Other generalizations about his manner:
-Jesus does not as a rule initiate dialogue or debate, nor does he offer to cure people.
-Jesus rarely makes pronouncements or speaks about himself in the first person.
Conclusion: The fact that these characteristics are present in multiple, independent sources (Mark, Q, Thomas, M, L, John) implies a single person to initiate these sayings. Given this information, it is rational beyond a reasonable doubt to consider Jesus to be a historical person.
For this post I am referencing The Five Gospels, by Robert Funk and Roy Hoover. If you want to investigate the sayings of the historical Jesus, pick up this book. Finally, before atheists on this forum ignore me as a fundamentalist Christian I should point out two things: 1) No one who supports the Jesus Seminar is a fundy. 2) I am not a Christian.
"A man can no more diminish God's glory by refusing to worship Him than a lunatic can put out the sun by scribbling the word, 'darkness' on the walls of his cell." (CS Lewis)
"A young man who wishes to remain a sound atheist cannot be too careful of his reading." (CS Lewis)