I'll be writing a devotional book on the Sermon on the Mount in the next couple months. Although the structure of Matthew 5-6 gelled with me a long time ago (rightly or wrongly), it's only been in the last week that Matthew 6-7 have. Although I use the Sermon on the Mount as an example of how to survey a text, I've always been a little less confident about the second half than the first.
Here's just a thumbnail of how I see the structure:
Matthew 5:3-16 Beatitudes
Kingdom Introduction--presents the values of the kingdom. It involves some contrasts that will play out throughout the sermon--between now and not yet, between the visible and the heart, between true blessedness and superficial blessedness.
Matthew 5:17-20 Key Verses
These verses are ground zero in the sermon. They are a general statement that plays itself out in the rest of it in two ways: 1) Jesus, as the new Moses, gives the fulfilled, the complete, perfected, and authoritative interpretation of the Law and the Prophets and 2) that fulfilled interpretation is true righteousness, a righteousness that exceeds that of the scribes and Pharisees. These verses also form an inclusio with 7:12, which returns to the theme of summing up the Law and the Prophets with the Golden Rule or what we know from Matthew 22 as the love commandment.
Matthew 5:21-48 The Fulfilled Law
In this series of contrasts, Jesus plays out what he means by fulfilling the Law. It's not merely the superficial, external rule but the playing out of the love principle in the heart in each area. In some cases, this shakes up and shuffles an OT law (e.g., eye for eye). The chapter climaxes with the general principle in 5:43-48. The fulfilled law not only loves ones friends but one's enemies as well. This is what it means to be complete or "perfect" as our heavenly Father is complete (5:48).
Matthew 6:1-7:12 True Righteousness
This section plays out 5:20 and what exceeding righteousness is in contrast to the righteousness of the scribes and Pharisees. First, their righteousness is only acting (6:1-18). It is external rather than a righteousness of the heart. It is a righteousness for now rather than for the kingdom. It is a righteousness for the world rather than for God and the kingdom.
They lay up treasures on earth (6:19-24). They judge others when in fact their heart is guilty of the same things (7:1-6). If they focus too much on receiving honor in the here and now, the visible (remember the topsy turvy values of the Beatitudes), one can also focus too much on the troubles of the visible and the moment. The worrier is also wrongly focused on the current mourning (cf. 5:4), on the current crisis before the kingdom comes (6:25-34).
So the person in current need, before the kingdom comes, should ask and seek from God, who is a loving Father (7:7-12; cf. 5:43-48). And we are to be complete as he is complete and do to others what we would wish them do to us (7:12).
Matthew 7:13-27 Be Wise!
The sermon now concludes with several warnings. The most central one is to be a wise builder. Someone might hear this sermon and do nothing with it. That's like a foolish builder. That person's house will fall. The wise builder hears the "rock" of the sermon and applies it.
Relatively few will do that. The gate to life is narrow. Even some of those who pretend to hear are only faking. There will be false prophets who, like the scribes and Pharisees, pretend to go along with the kingdom but whose heart isn't really with it. By the time Matthew was writing, this was probably a comment on people in his context.
Finally, like the Parable of the Sheep and the Goats in Matthew 25, not everyone who claims Jesus as Lord on the Day of Judgment will make it. Those who are only playing at it will be cast out into outer darkness, even though they may have done spectacular external things.