Many scholars (but not all) claim the New Testament manuscripts (Synoptic Gospels) were copied from one another and were probably written late in the 1st century CE—after the destruction of Jerusalem, or even early in the 2nd century CE. If we assume as reasonably true the tradition of the Christians of the 2nd century, that Mark is actually Peter’s Gospel, written by Mark who traveled with Peter, then we can see how both Matthew and Luke could be very similar to Mark in many places without actually having a copy of Mark before them. How so? Well, if, as is presumed by many, the Gospel accounts were an oral tradition for a number of years, then both Matthew and Luke could be very similar to Mark without their having a copy before them as they wrote their accounts. After all, would any American Christian really need a copy of the hymn, Silent Night, before him or her if one wished to write it out for a friend?
Therefore, if the presumption of oral tradition is true, all of the Gospel narratives could have been written independently of each other, and any one of them could have been written first, although even Christian tradition puts John last. Each writer, if he were not an Apostle, would certainly have had to interview the eyewitnesses and construct their accounts accordingly. Luke tells us in the first four verses of his account that he was diligent in tracing out his records to their sources. This would have to have included Peter (traditional source of Mark), whom all the Gospel accounts claim to have been a chief Apostle. Nevertheless, there is no reason to believe any Gospel narrative bearing the name and written by an Apostle wouldn’t have been corroborated by several other Apostles before copied for distribution.
Personally, I don’t hold to the oral tradition assumption (at least not in the manner that is presently assumed by many of its advocates), so I want to be upfront about that, but, even if the oral tradition is assumed to be exactly like many of its advocates perceive, why couldn’t things have occurred just as I argue above? Is there something in the accounts themselves that would belie my reasoning? If so, do you care to discuss it?