By the end of the second century, the Church Fathers had realised that there was a literary dependency among the synoptic gospels (Matthew, Mark and Luke) such that two of them must have been copied from the third. They decided that Matthew's Gospel was the first one, and that Mark and Luke were copied from it. This evaluation on the part of the Church Fathers has resulted in the continued belief, widespread among conservative Christians and many theologians, that Matthew's Gospel was the first to be written.
While biblical scholars agree that there was a literary dependency, they now realise that Mark's Gospel was the first of the New Testament gospels and that Matthew and Luke were copied from it. John's Gospel is believed to have been inspired by Luke's Gospel, with some material taken direct from Mark. This dependency process alone demonstrates that Mark's Gospel was written before the other New Testament gospels.
Biblical scholars say that the gospels were originally anonymous and were only attributed to the apostles whose names they now bear later in the second century. Scholars say that we do not know who really wrote the gospels, but they do tell us that they were written decades later than traditionally assumed. There is clear evidence in Mark's Gospel that it was written approximately 70 CE. In Mark 13:2 Jesus was said to prophesy the destruction of the Temple, an event that occurred in 70 CE. According to Mark, Jesus went on to predict the end of the world within the lifetime of his followers. If Jesus had really prophesied the destruction of the Temple, he would have been correct, but he would have been in error about the imminent end of times. Since it can not be accepted that Jesus made predictions that were capable of being in error, these prophecies must have originated with Mark, writing at a time when he would have known of the destruction or imminent destruction of the temple and when it really seemed as if the world was coming to an end. An apparent reference to the Book of Daniel indicates that it could not have been written later than the early 70s.
Biblical scholars have demonstrated that Matthew and Luke were based on Mark's Gospel with, for example, Matthew having some 600 of the 666 verses in Mark, often using the same words in the original Greek language. Luke's Gospel does not incorporate any material from Mark 6:47 to Mark 8:27a, a total of 74.5 verses that were probably on exactly 13 pages of Greek text missing from the copy of Mark's Gospel relied on by the author of Luke. This 'Missing Block' results in the curious conjunction found in Luke 9:18 "And it came to pass as he was alone praying, his disciples were with him: and he asked them ..." These clauses are more meaningful when found in Mark at the start and end of the Missing Block. This unambiguous evidence of copying establishes the sequence of authorship.
Other evidence, although of secondary importance, supports the conclusion that Mark was the first New Testament gospel. It is interesting of the early Christian writings that an anti-Jewish sentiment developed over the course of the decades. In his epistles, Paul says that he is proud of his Jewish heritage; Mark begins to show some antipathy to the Jews, or at least to their religious leaders; this is more pronounced in Matthew, and more so again in Luke. John's Gospel shows a strong dislike for the Jews. This helps establish the order in which the remaining gospels were written: Matthew, then Luke, then John.