So there are questions about when the gospels were written, which were written first, which are based on which etc.
“Traditionally, the gospel is said to have been written shortly before A.D. 70 in Rome, at a time of impending persecution and when destruction loomed over Jerusalem. … Modern research often proposes as the author an unknown Hellenistic Jewish Christian, possibly in Syria, and perhaps shortly after the year 70.” usccb.org/nab/bible/mark/intro.htm
The ancient tradition that the author was the disciple and apostle of Jesus named Matthew (see Matthew 10:3) is untenable because the gospel is based, in large part, on the Gospel according to Mark (almost all the verses of that gospel have been utilized in this), and it is hardly likely that a companion of Jesus would have followed so extensively an account that came from one who admittedly never had such an association rather than rely on his own memories. The attribution of the gospel to the disciple Matthew may have been due to his having been responsible for some of the traditions found in it, but that is far from certain.
The unknown author, whom we shall continue to call Matthew for the sake of convenience, drew not only upon the Gospel according to Mark but upon a large body of material (principally, sayings of Jesus) not found in Mark that corresponds, sometimes exactly, to material found also in the Gospel according to Luke. This material, called “Q” (probably from the first letter of the German word Quelle, meaning “source”), represents traditions, written and oral, used by both Matthew and Luke. Mark and Q are sources common to the two other synoptic gospels; hence the name the “Two-Source Theory” given to this explanation of the relation among the synoptics.
In addition to what Matthew drew from Mark and Q, his gospel contains material that is found only there. This is often designated “M,” written or oral tradition that was available to the author. **Since Mark was written shortly before or shortly after A.D. 70 (see Introduction to Mark), Matthew was composed certainly after that date, which marks the fall of Jerusalem to the Romans at the time of the First Jewish Revolt (A.D. 66-70), and probably at least a decade later since Matthew’s use of Mark presupposes a wide diffusion of that gospel. The post-A.D. 70 date is confirmed within the text by Matthew 22:7, which refers to the destruction of Jerusalem. ** usccb.org/nab/bible/matthew/intro.htm
Among the likely sources for the composition of this gospel (Luke 1:3) were the Gospel of Mark, a written collection of sayings of Jesus known also to the author of the Gospel of Matthew (Q; see Introduction to Matthew), and other special traditions that were used by Luke alone among the gospel writers. Some hold that Luke used Mark only as a complementary source for rounding out the material he took from other traditions. Because of its dependence on the Gospel of Mark and because details in Luke’s Gospel (Luke 13:35a; 19:43-44; 21:20; 23:28-31) imply that the author was acquainted with the destruction of the city of Jerusalem by the Romans in A.D. 70, the Gospel of Luke is dated by most scholars after that date; many propose A.D. 80-90 as the time of composition.usccb.org/nab/bible/luke/intro.htm
Critical analysis makes it difficult to accept the idea that the gospel as it now stands was written by one person. John 21 seems to have been added after the gospel was completed; it exhibits a Greek style somewhat different from that of the rest of the work. The prologue (John 1:1-18) apparently contains an independent hymn, subsequently adapted to serve as a preface to the gospel. Within the gospel itself there are also some inconsistencies, e.g., there are two endings of Jesus’ discourse in the upper room (John 14:31; 18:1). To solve these problems, scholars have proposed various rearrangements that would produce a smoother order. However, most have come to the conclusion that the inconsistencies were probably produced by subsequent editing in which homogeneous materials were added to a shorter original.
The final editing of the gospel and arrangement in its present form probably dates from between A.D. 90 and 100. Traditionally, Ephesus has been favored as the place of composition, though many support a location in Syria, perhaps the city of Antioch, while some have suggested other places, including Alexandria.
In light of this, the disagreement between the gospels on the matter of the founding of a Church becomes even more important: forums.catholic.com/showthread.php?t=97368 i