Here is an extract from the prestigious Oxford Dictionary of the Christian Church. The relevant entry is entitled The Gospel according to St Matthew. It reads in part as follows,
The Gospel is probably to be dated c. AD 80-90, and unlikely to have been written by an eye-witness. Most scholars hold that the author drew extensively on Mark, which he expanded with other sources, especially ‘Q’.
The Greek of the Gospel is correct, if rather colourless. Matthew avoids some of Mark’s undignified words and is more concise, but much of Mark’s subtlety and artistry is lost.
Christians tend not to know that Matthew (the actual disciple of Jesus) did not write the gospel attributed to him and assume, wrongly, that it is authentic eyewitness testimony. In reality it is written by an unknown second generation Christian. Whether the author gives us an accurate historical account of the life of Jesus has been the subject of much dispute. Virtually all Biblical scholars admit that the gospel contains at least some legend (see this) and made up material which in academic circles they typically and euphemistically call “secondary”.