Blessed are the pure in heart,
for they will see God.
Purity was an important issue for the religious in Jesus' day, and the Gospels record several incidents and sayings when Jesus dealt with their wrong perceptions about purity, and I've looked at some of these in earlier posts. For example, by refusing to wash His hands before a meal Jesus declared that we cannot be 'defiled' or 'contaminated' through our contact with people who do not meet our standards of purity. By allowing 'unclean' people to touch Him, and even by reaching out to them, Jesus actively demonstrated that He was welcoming into His Kingdom those who were 'impure' by the standards of others and even those who were previously 'impure' by God's standards revealed in the Torah!
Purity under the Jewish law had an outward emphasis - as a result the religious distanced themselves from others and shrank away from outside ('worldly') influences and relationships, emphasising 'separation'. However, in a saying recorded in Matthew 15:11 Jesus emphasised that defilement came from a person's heart: "What goes into a man's mouth does not make him 'unclean,' but what comes out of his mouth, that is what makes him 'unclean.' "
Similarly, in this beatitude, Jesus is saying that purity is not an outward thing but is 'in the heart'. Later in this sermon He was to go on to say that religious observances and practices which are designed to get the attention of others come from a wrong motive and are therefore useless. After speaking of God's deliverance of the oppressed (the poor, the hungry, those who mourn), He turns to speaks of our role in bringing deliverance ('show mercy' - or actively bring deliverance). In placing this beatitude here He is emphasising that our motive in helping the poor and oppressed must spring from 'a pure heart' and not for any hope of recognition or praise by others.
Jesus built on this in His next beatitude:
Blessed are the peacemakers,
for they will be called children of God.
One of the marks of the promised Kingdom of God is peace, and the prophets repeatedly spoke of peace as a major characteristic of the Age to Come. "On earth peace to men" was announced by the angels at the birth of Jesus, and Acts describes the Gospel as "the good news of peace through Jesus Christ" (10:36). Paul said of our salvation that "since we have been justified through faith, we have peace with God through our Lord Jesus Christ" (Rom 5:1) and said of Jesus "he himself is our peace" and "He came and preached peace to you who were far away and peace to those who were near" (Eph 2:14-18). Unlike the first 4 beatitudes which announced that God is about to bring deliverance, this beatitude concludes the a group of 3 which emphasise our role in bringing deliverance as Kingdom-people.
Jesus may well have had in mind the militaristic efforts of some Jews to overthrow the Romans. It's likely that two of the twelve disciples were previously members of underground 'liberation' organisations. However, rather than getting our needs met through the destruction of our enemies, Jesus taught the opposite. Ironically, it was a Roman soldier, someone who had previously been a target of the liberationists, who declared at the crucifixion "truly this was the Son of God" (Mark 15:39). As the Prince of Peace Jesus brought together those who would once have killed each other. As the children of God, and sons of the Kingdom, we are commissioned to not only preach a message of peace but to be peacemakers.
Even in our dealers with people who have different opinions, 'heretical theologies' or 'wrong doctrine', we must remember that we are called to be peacemakers and not separatists.