The Jewish Rabbis in the time of Christ were wont to use, and their students would be familiar with, the number seven as an outline, a method of teaching a specific type of lesson. They would make three points, one-two-three, and then restate those same three points in other words, four-five-six. Finally, they would draw the inescapable conclusion, seven. Jesus was familiar with this method of teaching and knew that his audience was familiar with this method of teaching. If we assume that he began his sermon in Matthew 5 with this approach, the lesson beginning his discourse becomes very powerful indeed.

Blessed are the poor in spirit, for theirs is the kingdom of heaven.
Blessed are they that mourn, for they shall be comforted.
Blessed are the meek, for they shall inherit the earth.

(Now the same three points in other words:)

Blessed are they which do hunger and thirst after righteousness, for they shall be filled.
Blessed are the merciful, for they shall obtain mercy.
Blessed are the pure in heart, for they shall see God.

(Now the inescapable conclusion:)

Blessed are the peacemakers, for they shall be called the children of God.

To Jesus' audience Matthew 5: 9 would conclude the lesson, make the point to be heard, and the rest of the paragraph (verses 10-12) would discuss the results of that point. In our day we would make the points and discuss them like this:

Blessed are the poor in spirit, those who hunger and thirst after righteousness. These are those who will seek after and enter the church and be filled with righteousness, themselves, for their efforts. They realize that they need something to fill the void in their spirit, their life, and they will seek to fill the void as one who hungers seeks without being prodded to fill his stomach, or as one who thirsts seeks to quench that thirst. One does not need to beg a hungry man to eat, nor does he need to force a thirsty man to drink. Just so, those who realize their poverty of spirit will not need to be forced to seek the church, the only place where their hunger and thirst can be satisfied. Once these poor souls satisfy their own void and are filled with the righteousness of God (2 Cor. 5: 19-21), they will begin to look to others around them and see the need of those others for the same nourishment they themselves have received . Blessed are they that mourn and show mercy to others. The ones who have entered the church and been filled even beyond what they had hoped will now begin to care for those loved ones who are hungering and thirsting, and as they mature they will begin to look beyond their own family and friends to the whole of mankind; they will mourn the condition of the world, the condition they themselves have narrowly escaped, and begin to show mercy by reaching out with the truth, the good news, of what they have found. At first, like children, their efforts will be self-willed and clumsy. Only their enthusiasm will cause some to listen, but for the most part little will be accomplished; but as they become meek and pure in heart, more and more under the control of God, more and more directed by their understanding of his word and his will, the more of the world they will inherit, the more of mankind they will be able to direct to God as they see him more and more clearly. These are they, says Jesus, who will enter the work of God and become peacemakers, not between men or even brethren caught in petty struggles in this life, but peacemakers between God and mankind (2 Cor. 5), ambassadors carrying the peace of God to a lost and dying world. These are they whom God will trust with his mission, the mission of his Son on the cross, the work of God their Father. God help us that, should we claim the title of Christian, we should live up to the great task that has been entrusted to us.