Among the most striking things about the way Pennington pitches the Sermon on the Mount is to suggest that whole-heartedness lies at the heart of the ethic that Jesus is teaching, and that this through-line makes sense of how to interpret the whole.
For my part, I don't intend in these few posts to do anything like a thorough exegesis of Matthew 5-7. I have notes on that sitting around, but it's not exactly the kind of musing I like to do here. Instead, what I propose to do today is think through the idea of whole-heartedness, as one of the foundations for understanding the Sermon, and then apply that thought in looking backwards from 5:48 through to the start.
5.48, "You therefore be whole as your heavenly father is whole."
This verse functions as a conclusion to the first large section of the Sermon (5.17-48) and is a restatement of Lev 20.26. Pennington says that he thinks Jesus has chosen to restate it in this language, rather than 'holy', precisely because of the way that holiness in the Pharisees' world had come to mean primarily external matters of purity. And this is what Jesus' interpretation of Torah challenges at every point. The 'righteousness' of "whole-person behaviour that accords with God's nature, will, and coming kingdom" (v20 For I say to you that unless your righteousness abounds more than the scribes and Pharisees, you will not enter the heavenly kingdom) that must exceed the Pharisees does so precisely in this whole-heartedness, and this is the idea that is seen throughout the 6 exegeses of 5.21-47.
In 5.21-47 Jesus offers an exegesis of six different elements of Torah: Murder, Adultery, Divorce, Oaths, Retaliation, and Love. Let's start at the end with Love, in 5.43-47. Here Jesus takes the Torah command: love your neighbour and its commonly understood (and practiced) corollary, hate your enemy, and explodes it from the inside by teaching love for enemies. It is fair to say that love for one another is prevalent and pervasive as the ethic of Jesus throughout the New Testament. Jesus' teaching pushes the boundaries of love to a universal extent. Whereas we, by nature, love people in our in-groups (however we define them: social, ethic, national, political, religious) and hate those in out-groups (foreigners, extra-nationals, political opponents, members of other religious affiliations), so much so that all humans do this, we (in Volf's language) choose whom to embrace, and practice exclusion of the Other.
It's easy to think that Jesus is the model of love for enemies, and while this is true it should not be thought so at the expense of God the Father. God the Father practices and models love for his enemies, seen through his redemptive love in sending his Son, and reconciling his enemies to him on the cross. So, too, in imitating both the Father and Jesus, we are to love our enemies, from the heart, which requires what? At the least it requires that we seek their genuine good in prayer. As many have recognised, it becomes increasingly difficult to hate those you pray for. Prayer is meant to be the expression of the innermost person, hence the call to pray for persecutors.
The failure to love enemies, places the Pharisees on the same level as tax-collectors and gentiles (v46-47). That is to say, failure to enact Jesus' love ethic means that however righteous we are externally, internally we are as righteous as those deemed 'least-righteous' in society, because they too love their in-group and hate out-groups.
We can observe, with Pennington again, that hypocrisy in Matthew's Gospel differs from hypocrisy in modern parlance. We say 'hypocrite' to mean someone who publicly affirms X but then actually does Y - e.g. the conservative politician who rails against homosexuality and enacts laws against it, but then is discovered to have been frequenting same-sex prostitutes. Jesus means something different - people who are externally righteous, doing all the right things, but their hearts are hollow, empty, far for committed to what they are doing. This is nowhere clearer than in the "Woes" of Matthew 23 (the antitheses to the "Blessings" of ch 5), and especially 23.27 woe to you, scribes and Pharisees - hypocrites! Because you are like whitewashed tombs which appear beautiful on the outside, but on the inside are full of the bones of the dead and everything unclean.
If we work backwards through the six exegeses, the same themes of whole-heartedness, hypocrisy, and Torah-fulfilment are seen. In the fifth, Jesus takes the Lex Talionis, which aimed to restrict retaliation, and shows its fullest intention : non-retaliation in the face of evil. There is a justice greater than seeking our own justice, and that is leaving it in God's hands. Instead, we should not resist an evil person. We should however resist evil. This is how we ought to read Jesus here, because the theo-logic of placing this text into conversation with the rest of Scripture pushes us towards a desire to win over the evildoer through loving suffering.
In the realm of speech, external, technical fulfilment of oaths is not the point of oaths at all. And so Jesus' treatment of Torah shows us that integrity of speech and fidelity of action are what underlies, and guides, Ex 20.7 and Lev 19.12. In the realm of marriage and divorce (which is dealt with in greater depth, and should be read in concert with Mt 19.1-12), Jesus' teaching is not so much about the casuistry of divorce law, as about heart-motives. A (1st century Palestinian) man who divorces his wife without due cause, shouldn't presume that he is free to remarry, precisely because divorce exists as a concession to hardness of heart. Fix the heart and (let's not be naive and imagine this magically fixes all the messy situations of the world, but) some of the driving motivations of divorce become thin.
The first two form a pair, treating what the Decalogue and Torah in general consider two of the most devastating transgressions which wreak havoc on human relationships and communities. The Torah isn't suggesting that it's 'good enough' to simply avoid adultery or murder, but Jesus' exegeses makes clear that the heart issues which are the seeds of these external behaviours are in view. Jesus certainly doesn't think it's okay to commit adultery if your heart isn't in it! But, to his point, Jesus' desires that his followers be the kind of people in which there isn't the heart-origin of adultery.
The answer to this isn't what is commonly called purity culture. I think purity culture is a way to try to regulate even further external human behaviour and cultural norms in order to manipulate and control people with rules and shame. Purity culture idolises marital sex, demonises extra-marital sex, and reduces people to sexual sinners. I posit that Jesus is suggesting something actually far more radical. It's this : that (men, especially) ought to practice the virtue of looking upon women as human beings and persons in the image of God, not as objects of desire. This shouldn't be radical at all, it should be basic human praxis, but we live in the world we live in.
Finally we return to the first of the exegesis, which loops back around to the sixth (or vice versa, if you prefer to read this text forwards), wherein Jesus takes us from the fruit (murder) to the seed (anger and hatred). The deepest sense of the prohibition against murder is anger against a fellow image-bearer, which is why this is also the paradigmatic post-fall sin, Cain's murder of Abel. Jesus gives this two practical applications, one inside the community and one outside. Inside or outside the community of faith, Jesus' people are to seek peace and reconciliation as the antidote to anger which destroys both relationships, and people themselves.
To put all of these another way:
If you wholeheartedly love your enemies, you'll pray for them and seek their good
If you wholeheartedly love your opponents, you won't retaliate but forgive
If you wholeheartedly love others, you won't have two standards of speech and honesty
If you wholeheartedly love your spouse whom you made a lifelong commitment to, you won't divorce them
If you wholeheartedly love people who aren't your spouse, you won't treat them as objects of sexual desire and possession
If you wholeheartedly love others, you won't be angry with them, speak ill of them or to them, or allow unresolved conflict to fester between you, let alone murder them!
To bring this already long post to a close, let's step backwards one more time to the main thesis statement of the Sermon in 5.17-20. How are we to understand the relationship between Jesus and the Torah? In what sense does Jesus fulfil it? Granted that there's a lot more to say about this topic, not least Paul's view, but here at least I think we can say that the Matthean Jesus in no way is setting aside the Torah (The Law as interpreted and explained by the Prophets), nor replacing it, nor giving an extra-difficult-version of it for Christians, but he comes to exegete the Torah in a way that brings it to its fullest expression and deepest meaning. Which is the whole-hearted righteousness we've seen in the six exegeses that follow, and is summed up in the command to be whole as the Father is whole, and in the universal ethic of love that has no boundaries, even redeeming, forgiving, and reconciling enemies.