This, however, should not trouble or dissuade kingdom-seekers, as Jesus says that “the kingdom of heaven belongs” (Matthew 5:10b) to those that endure such things. They should take heart and be encouraged, “rejoice and be glad… for they persecuted the prophets before you in the same way” (5:12). Not only can one see this worked out in the early church, as demonstrated by Luke’s historical treatment in the book of Acts and as Matthew undoubtedly has the widespread persecution of Jesus-followers (at the direction of the Temple authorities in Jerusalem) in mind as he delivers Jesus’ words, but one can also see that Jesus, though He does nothing more than live out His teaching on mercy, purity of heart, and the making of peace, is most certainly persecuted for the sake of the way that He insists upon demonstrating the Creator God’s covenant faithfulness---insulted and persecuted for the way in which He speaks on behalf of and represents His God.
Having laid out His premise, Jesus essentially goes on to explain what He means by His Moses-and-Abraham-mindful introductory statement, with His treatments of anger and murder, adultery, divorce, the taking of oaths, retaliation, love for enemies, giving, prayer, proper fasting, true and lasting treasure, worry, and judging. It is through His explanation that it is possible to go on to learn what it means to be poor in spirit, to rightly mourn, to be meek, to hunger and thirst for righteousness, to be merciful, and to be peacemakers. Most importantly, for the purposes of this study, here it is also possible learn what Jesus means when He speaks of being “pure in heart.” Since it must be the burning and overwhelming desire of the heart to “see God,” rightly assessing this statement could not be more crucial.
So if one understands that Jesus is going to take the time to explain what He means by His pronouncements in the beatitudes, then it is going to be necessary to look to the remainder of the sermon in order to discover Jesus’ ideas concerning purity of heart. Such is a relatively simple process, as one must merely look to instances of the use of “heart.” The first that to be encountered is later on in the fifth chapter. There Jesus is heard saying “You have that it was said, ‘Do not commit adultery.’ But I say to you that whoever looks at a woman to desire her has already committed adultery with her in his heart” (5:27-28).
Though Jesus speaks here of the heart, it seems to be more along the lines of a prevailing condition of the heart, rather than instruction that would reveal what it is that He means by being pure of heart. So though it is certainly instructive, and though it certainly informs denizens of the Creator God’s kingdom that more is expected of them (especially in light of the Resurrection, through which one would naturally view Matthew and the whole of Scripture), it does not truly assist a believer in learning how one can go about becoming pure in heart.
One does not achieve purity of heart, which is probably something that can be outwardly demonstrated in a tangible ways, by simply avoiding adultery or lustful desire. At the same time, avoidance of adultery is something that is completely expected, and nobody is congratulated for not committing adultery, whether it be physical or mental. One should not expect to get to see the Creator God simply because one did not travel that path. There must be something more.