The earliest Christians did not have any written records of the life and teachings of Jesus. During the course of Jesus' public ministry, no one felt the need to make a written account of what Jesus did or what he said. Those who were close to him could pass to others what they remembered about him. Those who looked upon him as the Messiah believed that he would soon inaugurate a new kingdom; all that was necessary to know about him could be remembered until that time. They were, of course, sorely disappointed when he was put to death on the cross, for it seemed as though his cause was lost. Afterward, they were convinced that despite his death, he really was the Messiah. With this recognition on their part, there was now a new reason for remembering the events of his earthly life. What he had done before his death took on a new meaning in relationship to what happened since that time. Evidence was needed that would convince unbelievers that Jesus was the Messiah, and the faith of those who already believed in him needed to be confirmed and strengthened. The early Christians believed that Jesus would soon return to earth and complete the work of preparing for the coming kingdom. An authentic record of his life on earth would be a great help to those who were expecting his return, and with the passing of the years, the need for such a record was greatly increased.
That the Gospels were not written until nearly forty years after Jesus' death often raises questions concerning their reliability. The situation is complicated also by the fact that the Gospels are not all alike, nor is it possible to harmonize completely all of the materials contained in them. They agree on many points but disagree on others. What has been called the "synoptic problem" concerns finding some hypothesis that addresses the origin of the Gospels and that accounts both for their agreements and for their differences. Many different solutions have been proposed, but no one of them is fully accepted by all New Testament scholars. The most widely held view maintains that the Gospels, in their present form, are based very largely on older source materials in existence at a time not far removed from the events that they record. If the authors of the Gospels used the same sources, the similarities between the Gospels would be explained; likewise, that other sources were used by only one of the authors would explain the differences that we find when comparing the different accounts. That the earliest source materials were written by people who were contemporaries of Jesus and his disciples adds considerable weight to their historical reliability.
The Gospel of Mark is generally agreed to be the oldest of the three Synoptic Gospels — Matthew, Mark, and Luke — and was used as one of the sources for each of the other two. The outline of events as they occur in Mark is followed by each of the other biographers, and about two thirds of the material found in Mark is also present in both Matthew and Luke. This similarity suggests very strongly, although it does not prove, that the authors of Matthew and Luke took their materials from Mark. There is also reason to believe that both Matthew and Luke had another source in common. Both of them report a considerable amount of Jesus' teachings in addition to what is contained in Mark. To account for this extra material, it is assumed that a document composed of Jesus' sayings was in existence and was another source for Matthew and Luke. Scholars refer to this other source with the letter Q, the first letter of the German word Quelle, which in English means source. Because there is some unique material in Matthew, possibly its author used still another source M, which was not used in any of the other Gospels. The same holds true for the Gospel of Luke, and scholars use the letter L to refer to his special source. This hypothesis concerning the origin of the Synoptic Gospels appears to be confirmed by the introductory paragraph found in the Gospel of Luke, which states that several lives of Jesus have been written and that Luke's author's purpose is to write a definitive biography about Jesus.