Some concern was felt in the early church over the existence of multiple gospels. This concern was by no means universal and many churches simply accepted the authority of all four gospel without great debate and therefore presumably without great problem. The very fact that a gospel was used tended to ensure that it was regarded as authoritative. However, attempts were made to solve what was evidently a problem to some. Some time around 150 CE Marcion proposed the gospel of Luke (with slight alterations) as the only authoritative text. However, he was excluded from the church of Rome. Tatian (c. 170 CE) constructed a "gospel harmony" omitting the parallels and harmonizing the differences.
What were these differences? The most striking non-agreements are between the Gospel of John and the three "synoptic" gospels. These differ in the matters of:
(1) The itinerary of Jesus.
John has Jesus visit Jerusalem three times, remaining there for more than six months on one occasion, whereas the synoptic gospels relate only one trip to Jerusalem when Jesus was arrested and crucified.
(2) The timing of Jesus' ministry.
John has Jesus preaching simultaneously with John the Baptist and carrying on a ministry of about three years, whereas the synoptics suggests a time of less than one year, starting after the imprisonment of John the Baptist.
(3) Jesus' form of teaching.
John portrays Jesus as preaching long discourses on specific topics, in the other gospels he speaks in short, rather disconnected sayings and in parables.
(4) The focus of Jesus teaching.
In John Jesus speaks frequently of his person, "I who speak to you am he," (John 4:26) or "I have come in my father's name but you do not receive me" (John 5:43) etc. In the other gospels there is no such emphasis on his person but rather on "the kingdom of God/Heaven" (Mat 21:43) (Mark 4:11) (Luke 6:20 ) See a Concordance for more examples, John's gospel mentions the kingdom a total of five times: 3:3, 3:5, 18:36 [x3])
(5) Episodes in the life of Jesus.
Few of the narratives of the synoptic gospels are repeated in John who tells many episodes unknown in the other three gospels.
Although these differences are striking they should not detract from the differences between the synoptic gospels themselves. The infancy narratives and genealogies in Matthew and Luke should be compared as should the ordering of their narratives. The different descriptions of John the Baptist in Mark and the two others should be noted.