Although purity can refer to ritual cleanliness, Jesus refers to a person who is actually pure in their inner being. The adjective pure (καθαρός) is the word used frequently in the Septuagint to translate “clean” in the Law. For example, Numbers 8:7 refers to “waters of purification” or clean animals (Gen 7:2). A Jewish worshiper going up to the Temple would wash themselves in one of the many pools leading up to the Temple entrances. This was a ritual performed to symbolize purity and the person could be said to have “clean hands” the waters could not make the person actually pure.
Is Psalm 24 in the background of this saying? R. T. France (Matthew, 204) and McKnight both suggest the possibility since the answer to the psalmist’s question “who may ascend the holy hill of God?” is the one with a pure heart.
In Matthew 5:8, the heart of the follower of Jesus is called clean. The heart is the center of a person in the ancient world so the one who is “pure in heart” (οἱ καθαροὶ τῇ καρδίᾳ) has been really been made pure. In Psalm 26:6-7, the worshiper says “I wash my hands in innocence and go around your altar, O Lord, proclaiming thanksgiving aloud, and telling all your wondrous deeds.” Similarly, 2 Timothy 2:22 uses the same phrase, those who call on the Lord from a “pure heart.”
What is a “pure heart”? In the immediate context of the Sermon on the Mount, the followers of Jesus are the ones who keep the Law internally. In 5:21-26 the follower of Jesus not only avoids murder, but also controls their anger and inner thoughts. In 5:27-30 they not only avoid adultery, but control their lustful thoughts. In both cases, the inner life of the true disciple of Jesus is pure.
The followers of Jesus also stand in contrast to the hypocrites, those who do their acts of worship in public to be seen by people (Matthew 6:1-18). The actions of hypocrites make them to be pure in heart, but in fact “are like whitewashed tombs” (Matt 23:27).
The result in the beatitude is remarkable: they will see God. Exodus 33:20 says no one can see the face of God. After God allows his glory to pass by Moses, God himself writes the Ten Commandments on two stone tablets and then he announces he is the gracious and compassionate God (Exodus 34:5-7). Yet Moses himself cannot see God. In Isaiah 6 the prophet sees the throne room of God and glimpses only the train of God’s robe. Yet he says ““Woe is me! For I am lost; for I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; for my eyes have seen the King, the LORD of hosts!” (Isa 6:5).
In contrast to these two examples of Moses and Isaiah, the true disciple of Jesus will see God. It is possible Matthew intends for us to remember the sign of Emmanuel (Matt 1:23), Jesus is “God with us,” but perhaps the force of the metaphor is judicial. In Psalm 11:7 “the Lord is righteous and loves righteous deeds, the upright shall behold his face.” Remember righteousness is not a state of inner holiness, but real social justice (Matt 5:6). Therefore the person who acts justly will behold God’s face is a metaphor for vindication before a judge. When Joseph interpreted the dream of the butler, he said the pharaoh would “life up his head” and render justice (he would be restored to his position and the chief butler). In the context of a trial, for an ancient Near Eastern king to allow someone to look up is a sign the person has been found innocent.
Once again this beatitude has some persecution and a (future) vindication of the persecuted followers of Jesus. The ones who are pure in heart (the disciples) will look upon the face of God and be vindicated when the king renders justice. There is eschatology here as well, since seeing God may hint at the future coming of the son of Man to render justice when he establishes his kingdom.