The four gospels of the New Testament were originally written anonymously. They were only later given the names: Matthew, Mark, Luke and John. Scholars do not believe that these are eyewitness accounts. And the gospel of Thomas, which is not included in the New Testament, maybe the oldest of all of them (60 CE).
The first time we have a list of the four gospels is in the Muratorian Canon that was written as early as the late 2nd century or as late as the fourth century according to Helmet Koester. The earliest fragments exist from about the middle of the second century. So the gospels must have been written before then.
Most scholars date the gospels from the end of the first century. But this is based upon the presupposition that they are historical biographies. As the search for the historical Jesus demonstrates, they are not historical biographies. They are either elaborations of a historical Jesus or a allegorical myth.
Now if we lay aside the presupposition that they are historical biographies, a more reasonable dating puts the Gospels sonetime in the beginning to middle of the 2nd century. Let’s say about 30 years later than the current scholarly consensus.
This gives us the following approximate dates, The early date is the scholarly consensus, the late date is my estimation;
Early Late Mark 70 100 CE Matthew 95 115 CE Luke 140 170 CE* John 100 120 CE
* The Gospel of Luke, that we have, seems to have been edited into its final revision in 170 CE. It was dedicated to the bishop Theophilus of Antioch. We know he became bishop in 170 CE and died ten years later. It was probably edited and dedicated to him to celebrate his ordination. Marion confirms this, saying that Luke was corrupted. Later writers turned the tables and blamed him for editing Luke. What evidence exists supports Marion’s contention