Gospels are a unique literary type. We easily see them as biographies (why wouldn't we?) and assume their purpose is to bring us information about historical events (which prove Jesus is God). This, of course, has some element of truth. The stories are events from the life of Jesus and the church. Jesus is the Son of God/God the Son and the Gospels are a testimony which invites us to believe. But they aren't a biography in the modern sense. The point is not to provide us with a birth to death story of Jesus' life. They are Gospel--Good News--about the Kingdom of God and Jesus' role as Messiah King.
Matthew, with his own purposes, under the inspiration of the Holy Spirit, provides us with an account of the Gospel. Scholars assume he used Mark as a basis and added another source, a collection of sayings (in common with Luke) as well as his own sources either oral or written. In addition, the times in which he lived (20 to 50 years after Jesus) influenced how the message is transmitted. The changes he makes to Mark reflects this. One difference is that Mark says "Jesus taught" and Matthew has provided us with numerous chapters of the actual content of the teaching. The relationship of the church to Judaism also changed over the decades. It was more a Gentile church by the close of the first century than a Jewish sect.
Mt 17:1-8 The Jewish Scriptures contain extensive narratives about Moses, and one hears echos of that here. In fact, Moses and Elijah (Torah and Prophets) are identified. This remind us that Jesus is a Jewish Messiah and the New Testament is an extension of the First Covenant(s). The light shining out of Jesus is not an uncommon thing--God manifests as light frequently with holy people. The metamorphoo (changed into another form, transfigured) is a revelation of Jesus' inner being. Daniel's prophecies about the (one like a) Son of Man are found here as well. The Voice explains that Jesus is the beloved Son (remember the baptism, remember God to Abraham about Isaac), however, unique to Matthew, is the declaration "with whom I am well pleased." Rather than "rabbi" (per Mark) Peter calls Jesus "Lord" here. In addition, only Matthew says that the apostles fell on their faces, and Jesus touched them and said "Rise and have no fear." Such additional elements remind us that --if we meditate on the event--we will understand the awe-some (and frightening) nature of this event.
17:9-13 As they come down the mountain (an image and metaphor much used to contrast a "spiritual high"--like retreats--and "real life"--the grind of every day) we learn that Jesus calls this a "vision" (horama. only time used in a Gospel; quite frequent in Acts about Peter and Paul). What exactly happened is hard to know (was it something that everyone could have seen or was it limited to just the three? Apparitions of Mary have frequently been limited to the children who see her, while others in attendance may "experience something" but do not see her themselves). The theological discussion about Elijah is further clarified in Matthew. He states that it is John the Baptist. Christian depictions of John (his clothing) are an overt illustration of this. Matthew also adds a prediction of the Lord's passion here. The glory is not without the cross. (However much happy-clappy spirituality would imply that it is joy and abundance all the time) I prefer the bright lights to the bloody suffering, how about you?
17:14-21 "Reality" hits us at the bottom of the hill. A man comes to Jesus (Matthew has him kneel--notice Matthew wants his reader to understand who Jesus is, His exalted status, by making shifts to Mark's account). It is an epic fail by the apostles who are unable to cure/exorcise the man's demon afflicted, epileptic son. Jesus' reaction is harsh. Matthew (and Luke) both add the word "perverse" to Mark's original "faithless" (which may be an intentional allusion to Moses in Deuteronomy 32:5) This is not characteristic of Matthew, who generally replaces Mark's "faithless" with "ones of little faith." What makes this so hard to meditate on is the sorry state of the church in its healing ministry in the industrial West. Our healing team has certainly struggled with this, and most churches either don't even try, they preach against it (saying "healing ended with Jesus and the apostles"---ignoring church history and the thousands of cases which prove that wrong!) Matthew, as is his tendency, omits much detail from Mark's account. Jesus heals the boy and he is "cured instantly" (in Mark the boys convulses terribly and becomes like a corpse, the people say he is dead and Jesus takes his hand, lifts him up and he arises). The baffled apostles ask, "Why couldn't we do it?" In Mark Jesus' answer is that there needs to be prayer. In Matthew it serves as an illustration of the problem. You have "little faith" (Matthew) but he also uses the statement about faith the size of a mustard seed. Ironically, the mustard seed is called the smallest of all seeds in a parable. The statement is found also in Mt 21 when Jesus curses the fig tree. Small faith may be a problem for healing, but it is enough to raise a mountain and throw it in the sea! The healing ministry, I think, is the great divider between big and little faith. We need to believe more and act as true apostles of Jesus! Note in Mark the emphasis is on the (supplicant) father's faith in Jesus to heal (and a prayer for more faith) while here it is on the disciples' lack of faith as ministers.
Mt 17:22-27 The second prediction of the His death and resurrection make clear the centrality of that. Matthew emphasizes the apostles' distress (against Mark's "confusion" and "fear"). The "expectedness" of the cross and resurrection is an important part of the story. It illustrates God's plan.
In tandem with this is one of the more "amazing" stories of Jesus. He knows from afar that Peter (note Jesus calls him Simon) was approached about the failure to pay the half shekel Temple tax. It becomes a Christological illustration (as Son He should not pay His Father/King). However, Jesus does not want to "scandalize/cause to stumble" so He has Simon Peter fish up a miracle! This seems to indicate Jesus has no money. The tension of fidelity to the Jewish faith and the freedom of Jesus are in play here.
Mt 18:1-14 We see the issue of scandal coming up again in this section. It is thought that some of the sayings of Jesus were grouped thematically and perhaps that is why this section has a bumpy feel... The apostles ask Jesus who is the greatest in the Kingdom? In Mark the disciples are arguing with one another about who is the greatest? Matthew's streamlined version emphasizes the child more. Children were loved in Jewish homes, but it was hardly a child-centered society like ours can tend to be (in family values areas). A child has no more status than the poor, the sick, the outcast and the folks whom Jesus seems intent on saving! It is hard for successful middle class types to understand the radical nature of Jesus' message. We prefer to be assertive and powerful. Jesus illustration with the child, I think, helps us understand what "saved by faith alone" really means. It is when we feel smallest and most helpless that we must rely on God. For most of us, God is Plan B (or C). We trust God , but trusting is easiest when we feel power and control over our lives. Jesus says greatness is in abject weakness. But what else would a crucified Messiah think?
The child-scandal connection is made in what follows. Jesus is very protective of the poor and weak and makes clear that those who cause them to fall are better off dead. How often do we, in our righteousness and education, trample on the faith of "little ones" which don't measure up to our standards?