If we understand that Jesus is going to take the time to explain what He means by His pronouncements in the beatitudes, then we must look to the remainder of the sermon in order to discover Jesus’ ideas concerning purity of heart. Such is a relatively simple process, as we must look to instances of the use of “heart.” The first that we encounter is later on in the fifth chapter. There we hear Jesus say “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 God’s kingdom that more is expected of them (especially in light of the Resurrection, through which we naturally view Matthew and the whole of Scripture), it does not truly assist us 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 physical or mental. One should not expect to get to see God simply because one did not travel that path. There must be something more.
Now it can certainly be said that one that engages in adultery, whether through the actual, physical action or through the desire of which Jesus here speaks, would not be spoken of as being pure in heart. To that we would add that this does not really take us any further down the path towards seeing God, which will be the lot of those that are pure in heart. Again, Jesus is making some programmatic statements, which would imply that He has a program in mind. Likewise, Matthew presents Jesus programmatically, so it behooves us to allow the sermon to build on its statements internally, while also looking for the way in which the sermon works itself out in application to the entirety of the narrative that is on offer in the whole of Matthew.
Before moving on to the next “heart” statement, let it be noted that there are many uses of “heart” throughout Matthew’s Gospel, and all of them escape the lips of Jesus. However, it is reasonable to presume that the explanation of what it means to be “pure in heart” is to be found within the sermon, as Jesus goes on to define His own terms within this bracketed context. Here, we must also consider the possibility that Matthew has taken teachings of Jesus that were offered up in various times and places, and grouped them all together into this one “sermon,” especially if he was desirous of highlighting Jesus as a Moses-like figure, thus fulfilling the Deuteronomic insistence that another prophet would arise, like Moses.
A reinforcement of such a notion, though we can also consider that Jesus spoke what we hear from Him in Matthew on numerous occasions, including this one, comes from the fact that Luke has Jesus saying much the same thing (though quite a bit less than Matthew) on a plain, rather than from a mountain. Along with that, if we look to the Gospel of Mark, and again considering that Jesus can indeed say these things on more than one occasion (which is quite the reasonable and probable proposition), then we see a fair number of Jesus’ pronouncements, mountain-related in Matthew, scattered throughout Mark’s narrative. So if it is the case that Matthew has purposely grouped together these words of instruction from Jesus, remembering that they, regardless of the format in which they are presented, present Jesus’ conception of the life of the citizens of God’s kingdom, then it is all the more important to allow for an internal consistency, with terms defined by the sermon itself. This is what we are attempting to do with the term “pure in heart.”
Now, having mentioned Deuteronomy in the context of Matthew’s desire to present Jesus as a lawgiver Who is like Moses but, in fact, superior to Moses (“you have heard that it was said… but I say”), it is worth going there to review what it was that was reported to have been spoken by Moses. Now, whether or not Moses actually said these things, and there is little reason to dispute this (though some do), such would not change the fact that Israel understood itself and defined itself according to its historical narrative (which included Deuteronomy), and especially that of the exodus and its attendant events (Sinai, the giving of the law, the wilderness wandering, entrance into the promised land, etc…). Upon our arrival there we find “The Lord your God will raise up for you a prophet like me from among you---from your fellow Israelites; you must listen to him” (18:15). This is then confirmed by the voice of the Lord, Who says “I will raise up a prophet like you for them from among their fellow Israelites. I will put My words in his mouth and he will speak to them whatever I command” (18:18). In between the two insistences that such will take place, which Matthew must simply have in mind as he constructs his theologically-tinged biography of Jesus, Moses says “This accords with what happened at Horeb in the day of the assembly” (18:16a). So when Moses speaks about the prophet like himself, he actually connects it to what took place on a mountain. We are then unsurprised to hear Jesus speaking from a mountain, which makes the Moses-related point even more forcefully.