An important factor is recognizing the people present can be divided into two groups:
Seeing the crowds, he went up on the mountain, and when he sat down, his disciples came to him. (Matthew 5:1) [ESV]
When Jesus looks at His audience, He sees those who are His disciples and those who are not. Then one should consider if some statements were addressed specifically to disciples. For example, the last "blessing" in the beginning is addressed to His disciples:
“Blessed are you when others revile you and persecute you and utter all kinds of evil against you falsely on my account. (5:11)
Those present who were not a disciple hear the message and yet were in no danger of persecution "on my account."
On the other hand, the Sermon begins by addressing everyone in the audience:
“Blessed are the poor in spirit, for theirs is the kingdom of heaven. (5:3)
The Beatitudes are delivered using the 3rd person "they" (αὐτός). Then Jesus shifts to the 2nd person "you" (σύ).
1 At this point the audience knows He is no longer addressing everyone, rather He has singled out His disciples for next part of the Sermon.
In considering the entirety of the Sermon, this technique is two-fold. First, His disciples are told they have a higher obligation than simply to obey the Law (e.g. 5:21-30). Second, those who are not His disciples hear the cost and requirements of being a disciple (e.g. 5:11-16, 5:21-30) and the consequences of failing to become a disciple (e.g. 7:21-27).
Chiasms and Chiastic Structure
Chiastic structure, or chiastic pattern, is a literary technique in narrative motifs and other textual passages. An example of chiastic structure would be two ideas, A and B, together with variants A' and B', being presented as A,B,B',A'.
Chiastic structures were commonly used in many ways during this time:
…the pattern is a widely employed pattern in the literature of antiquity. It is especially interesting to note the affinity of the structure of Rev. with that of a Greek drama... According to the compositional rules of tragedy, the climax falls near the center of the action, and the denouement comes near the end. The narrative poetry of republican Rome follows the same compositional rules. Students of the literature of Israel and Judaism have found the same structural pattern. The pattern is also present in the visual art of the time. Two examples appear to be especially interesting for the understanding of Rev. Two Roman coins of 35-36 C.E. bear images of the temples of Divus Augustus and Apollo. These temple images exhibit the balanced structure ABCDC'B'A'.
As this was a common device, the shift in pronouns at the beginning would create the expectation the same shift would be used at the ending, and that is exactly what happens:
Jesus begins by teaching:
A: using the 3rd person (5:3-10) [to everyone]
B: using the 2nd person (5:11-16) [to His disciples - present & future]
Jesus ends by teaching:
B': using the 2nd person (7:13-20) [to His disciples - present & future]
A': using the 3rd person (7:21-27) [to everyone]
Jesus concludes using the same technique. The audience not only knows the Sermon is coming to an end, they should connect the two pairs as shown above: those in the 2nd person (B, B') were addressed to His disciples and those in the 3rd person (A, A') to all people.
Thus the Beatitudes must be understood with 7:21-27 in view. Which is to say, none of those blessings spoken of in the beginning, however desirable, by themselves will ensure eternal life. One must also be His disciple or He will say to "them" He never knew "them;" "they" didn't build "their" house on His words. The implication of discipleship is in the background and creates a tension for those who are not: "Does this apply to me? Do I want to become a disciple?" On the other hand, those who are disciples "hear" an uninterrupted message which they know applies to them.
The Central Idea
When a chiastic structure includes a main point it is called a chiasm, which Brad McCoy defines as "the use of inverted parallelism of form and/or content which moves toward and away from a strategic central component."
4 He explains:
…since chiasm involves the parallel inversion of corresponding components in a particular discourse, resulting in an overall structural balance revolving around the distinct central component of the overall unit, a recognition of chiastic structure leads the interpreter properly to appreciate the pivotal function and the emphatic importance of that central thought unit.
Since there are pairs of supporting points, the central theme is not repeated. The consequence of this structure is to balance the central point between an equal number of supporting points:
A, B, etc ("before" supporting points)
FSP: Final supporting point "before"
X: A unique topic - the central point
FSP': First supporting point "after" (similar or antithetical to "FSP")
A', B', etc ("after" and "repeated" supporting points)
The inverted structure ensures the first "repeated" supporting point is the closest to the central point. This allows a listener to understand the "second half" of the message with this point clearly in focus and to reconsider their initial assumptions and understanding of the "first half."
The Lord's Prayer
If the Sermon on the Mount was delivered using the a chiastic structure with a unique central point, the obvious candidate for the main theme is "The Lord's Prayer." In fact, the passages immediately before and after "The Lord's Prayer" display the characteristics of a chiastic structure:
5 “And when you pray, you must not be like the hypocrites. For they love to stand and pray in the synagogues and at the street corners, that they may be seen by others. Truly, I say to you, they have received their reward. 6 But when you pray, go into your room and shut the door and pray to your Father who is in secret. And your Father who sees in secret will reward you. 7 “And when you pray, do not heap up empty phrases as the Gentiles do, for they think that they will be heard for their many words. 8 Do not be like them, for your Father knows what you need before you ask him. (Matthew 6)
The "Lord's Prayer"
14 For if you forgive others their trespasses, your heavenly Father will also forgive you, 15 but if you do not forgive others their trespasses, neither will your Father forgive your trespasses 16 “And when you fast, do not look gloomy like the hypocrites, for they disfigure their faces that their fasting may be seen by others. Truly, I say to you, they have received their reward. 17 But when you fast, anoint your head and wash your face, 18 that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you.
Regardless of whether 6:7-8 and 6:14-15 are considered part of the central message, what is traditionally called "The Lord's Prayer" has been placed between a pair of supporting points: "...do not be like the hypocrites because "...your Father who sees in secret will reward you."
Here is one possible outline for the Sermon on the Mount:
A Blessed are those who live "above" the Law (5:3-10)
B Disciples do good works despite persecution (5:11-16)
C Jesus will fulfill the Law (5:17-20)
D Greater righteous than the Law (5:21-48)
E Acts of greater righteous (5:43-6:4)
F Acting in greater righteousness in secret (6:5-8)
X The Lord's Prayer (6:9-13)
F’ Acting in greater righteousness in public (6:14-18)
E' Rewards of greater righteousness (6:19-24)
D' Living with greater righteous (6:25-7:11)
C' How others are treated sums up the Law and the Prophets (7:12)
B' All will be known by their deeds (7:13-20)
A' Blessed are those who act upon the Law Giver's words (7:21-27)
This outline explains why the points are in the order they are; it also helps to understand them. A chiastic structure which places the Lord's Prayer at the center of the Sermon on the Mount makes the will of the Father and the coming of His Kingdom a key theme. Thus, Jesus did not come to do away with the Law but to fulfill it by doing the Father's will, bringing the Kingdom to the earth. Also, for one to have greater righteousness than the Law (or the scribes and Pharisees), is the will of the Father, which the Law is only a partial expression. So, "do not murder..." falls short of the Father's will if one has hatred, even when there is no murder.
As chiastic structures were used both in oratory and written communication, there is no reason to assume Matthew patched together multiple teachings to make a single message. Rather, it is reasonable to conclude Jesus delivered a single message in the chiastic form recorded. While the central theme is bringing the Kingdom by doing the will of the Father, the role and question of discipleship permeates the Sermon.
The Kingdom and the Father's will are a type of inclusio to the Gospel:
From that time Jesus began to preach, saying, “Repent, for the kingdom of heaven is at hand.” (4:17)
And going a little farther he fell on his face and prayed, saying, “My Father, if it be possible, let this cup pass from me; nevertheless, not as I will, but as you will.”...Again, for the second time, he went away and prayed, “My Father, if this cannot pass unless I drink it, your will be done.”...So, leaving them again, he went away and prayed for the third time, saying the same words again. (26:39, 42, 44)
And the ending makes explicit what was implied in the Sermon:
Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.” (28:19-20)
The question of discipleship in the background of the Sermon on the Mount becomes a direct command to His disciples.
1. The Received Text has ὑμᾶς, this variant is also in the 2nd person.
2. Chiastic structure
3. Elisabeth Schüssler Fiorenza, The Book of Revelation: Justice and Judgment, Fortress Press, 1985, p. 176
4. Brad McCoy, Chiasmus: An Important Structural Device Commonly Found in Biblical Literature, Chafer Theological Seminary, 2003, Vol. 9 No. 2, p. 18
5. Ibid., p. 31