The authors of Luke and Matthew worked independently of each other without being aware of the other's writing. Both had borrowed portions from two earlier Gospels. They had added this material to their own writings, sometimes word for word. This would be prohibited today because of copyright restrictions; however, customs were very different during the 1st century CE; copying was quite a common practice and not considered unethical.

In addition, both Matthew and Luke incorporated additional material from their own unique sources. These are generally referred to as "M" and "L."

Further analysis showed that one of the source documents was the edited, public version of the Gospel of Mark. The other source document was an unknown "Sayings Gospel." As the name implies, this would consist primarily of elements from Christ's sermons, along with instructions, parables, witty comments etc. It would contain little accompanying narrative about the activities of Christ and his disciples. A total of about 225 verses of Luke and Matthew come from this source. The German researchers who pioneered in this work called this lost document "Quelle" which means "source". This is usually abbreviated to "Q".

There were some initial criticisms of the "Q" concept. The texts of many Gospels have survived - both the four in the official canon, and dozens more that never made it into the Bible. However, no copy of "Q" had survived intact into the modern era. Also, all known Gospels were written as narratives; no other Sayings Gospel had ever been seen; it was an unknown gospel construct. But after the 1945 discovery of the Sayings Gospel of Thomas at Nag Hummadi in the 1940's, the theory of "Q" became much more believable.

Two groups, the International Q Project and the Q Project of the Society of Biblical Literature were formed to promote research into the lost Gospel.

Many non-conservative theologians conclude that Q was written by unknown authors, starting circa 50 CE. It was widely used by one of the groups who followed the teachings of Jesus. The author of the Gospel of Thomas, a very early Gospel which never made it into the official canon, is believed to have quoted from Q as well.

Many religious liberals believe that Q represents the beliefs of an early group of Jesus' followers, when Jesus was considered a philosopher-teacher. This is before many theological concepts had been developed and incorporated into the Christian belief system: of Jesus being part of the Trinity, of church organizations, salvation, baptism, virgin birth, resurrection, etc. One author suggests that Q might be "a kind of missing link between the Jewish world of Jesus and the early Christian church." ₃

Other theologians of various persuasions are not convinced of the existence of Q as an early gospel. After all, no copy of Q has survived. No commentator from the early centuries of the Christian church refers to it. Some postulate that the author of Luke knew of, and copied from, the Gospel of Matthew. This would eliminate the need for Q.

There are some indicators that the authors of Matthew and Luke may not have worked independently. _4,5 For example: