Jesus summons those who would be his followers to radical devotion and radical dependence on God. His followers must be meek, must not retaliate, must go beyond the letter's law to its spirit, must do what is right when only God is looking, must depend on God for their needs and pursue his interests rather than their own, and must leave spiritual measurements of others' hearts to God. In short, true people of the kingdom live for God, not for themselves. (My overall approach to the Sermon on the Mount combines some approaches, but still remains one among many. For a more complete summary of various views on this sermon's message, see, for example, Guelich 1982:14-22; Cranford 1992; Allen 1992.)
Readers should contemplate the message of this sermon. Having summarized Jesus' message as repentance in view of the coming kingdom (4:17), Matthew now collects Jesus' teachings that explain how a repentant person ready for God's rule should live. Only those submitted to God's reign now are truly prepared for the time when he will judge the world and reign there unchallenged. This sermon provides examples of the self-sacrificial ethics of the kingdom, which its citizens must learn to exemplify even in the present world before the rest of the world recognizes that kingdom (6:10).
To be faithful to the text, we must let Jesus' radical demands confront us with all the unnerving force with which they would have struck their first hearers. At the same time, the rest of the Gospel narrative, where Jesus does not repudiate disciples who miserably fail yet repent (for example, 26:31-32), does season the text with grace. Most Jewish people understood God's commandments in the context of grace (E. Sanders 1977; though compare also Thielman 1994:48-68); given Jesus' demands for greater grace in practice (9:13; 12:7; 18:21-35), we must remember that Jesus embraces those who humble themselves, acknowledging God's right to rule, even if in practice they are not yet perfect (5:48). Jesus preached hard to the religiously and socially arrogant, but his words come as comfort to the meek and brokenhearted.
Of course one also needs to read grace in light of the kingdom demands; grace transforms as well as forgives. Jesus is meek and lowly in heart to the broken and heals and restores the needy who seek him; it is the arrogant, the religiously and socially satisfied, against whom Jesus lays the kingdom demands harshly (compare Mt 23).
Although the sermon's structure does not fit some modern outlines, it reflects a consistent pattern. Matthew gathers a variety of Jesus' teachings on related topics that appear in the source he shares with Luke. Ancient writers exercised the freedom to rearrange sayings, often topically; sometimes they also gathered sayings of their teachers into collections. Evidence within the sermon itself suggesting various audiences (5:1; 7:28) may also support the view that the sermon is composite. Scholars debate its precise structure, but 5:17-48, 6:1-18 and 6:19-34 are its largest complete units.