Geza Vermes had done us an immense favor by way of five excellent volumes on Jesus. The Authentic Gospel of Jesus is his fifth and it is a treatment without peer. While E.P. Sanders and Bart Ehrman both offer excellent short introductory volumes, Vermes book is unsurpassed in its unique approach to the subject matter. Rather than writing a biography of his subject, he has taken all words and ideas attributed to him and examined them for authenticity in order to discover the personality, religion and outlook of the historical figure of Jesus.
It is a perilous task he undertakes. Believers place primacy on faith, not on reasoned arguments and on religious truth rather than historical fact, and one does not have to search very far to find works by Vermes and others roundly condemned for challenging this faith merely by their questioning the inerrancy of the Gospels. Nevertheless, Geza Vermes has performed the task with brilliance. Contained within this book is every saying attributed to Jesus found in the Synoptic Gospels. He has divided them into categories by subject matter, for instance, prayers, parables and words of wisdom. He then subjects each saying to close scrutiny by placing it into the context of 1st century Jewish custom, law and thought as well as historical chronology. He also compares the forms found in Matthew, Mark and Luke and to the doctrines and beliefs of evolving Christianity, whether Jewish or the Gentile.
Some people have criticized Vermes' use of the Talmud and other rabbinic literature in comparing Jesus' utterances to those of later rabbis, but as he has pointed out, this is a useful way to determine how closely Jesus' words fit into Jewish thought since this material preserves older teachings dating from the time of Jesus or even earlier. Nor is this material "medieval" as has sometimes been charged; it dates from 200-500 CE.
Finally, if the writings of the Apostolic Fathers and other later Christian writers can be used to shed light on the early days of the Church (and they have been) why then should Jewish sources be ignored, particularly in light of the fact that Jesus was himself a Jew. In his lifetime, there was no Church, no doctrine, no dogma, no Christian theology, and indeed, no Christianity. As the Gospels date from a time when all these things had come into being, it is absolutely critical that they be subjected to the sort of scrutiny Vermes brings to bear.
Obviously, this is not the book for someone who is married to the idea of Gospel inerrancy, but for everyone else, it is a must have.