I intended to talk about why the author of Matthew must have been one of the twelve disciples. Most people believe that the author used Mark as the source for his narrative, and that he could not have been Matthew, because it would be unlikely that an apostle would rely on an account written by someone who wasn't an apostle.
But Matthew's narrative has its own distinct structure, such that that it cannot have been derived from Mark. And each section of his teaching material is set in a definite context within the narrative, and follows naturally from that context. Additional material has been added to both the narrative and teaching material either by the original author or a later editor.
The contents of the pre-edited text of Matthew show signs of direct association with Jesus during his ministry.
For example, Mark states simply that after John the Baptist was arrested, Jesus came to Galilee and began to preach. Matthew however gives the reason for Jesus' move to Galilee, saying that "When Jesus heard that John had been imprisoned, He withdrew to Galilee" and "From that time on Jesus began to preach". Then, after John's execution, Mark, like Luke, explains Jesus' withdrawal from the public eye as due to a need for rest, whereas Matthew says that it was because Jesus heard about John. In both instances Matthew links Jesus' withdrawal to Herod's actions towards John. This is what we would expect if the author was one of the twelve disciples, who would have had this kind of inside information about Jesus' ministry, in contrast with Mark for whom that detail was apparently not important. For another example, the author of Matthew (but not Luke or Mark) mentions flute players at Jairus' house when Jesus arrived there, typical of eyewitness recollection.