Three things right off the bat:
And then a fourth thing: You know (and depending on how the reading is announced or displayed in worship materials the congregation also knows) that this pericope was constructed by omitting some verses from a longer Bible passage. I'm not going to quibble about those omissions, but I do want to note two effects.
Because the reading starts with a confrontation over defilement and then ends with Jesus teaching about real defilement, hearers are going to hear that this passage is about defilement. This is a homiletical challenge. We hearers will "get" that the sins listed in verses 21-22 are motivated by "evil intentions from our hearts" and are "bad." But because "defilement" (i.e., ritual uncleanness) is not a prominent living concept in our culture, it's hard to guess whether hearers will be gripped in their souls by the idea of "being defiled." And the first omission (of verses 9-13) has the effect of intensifying this focus on defilement.
Jesus' accusation to the leaders, which includes a quote from Isaiah, raises the overarching prophetic concern that God's people are neglecting core issues of mercy because they are consumed with getting "human traditions" right. His tirade against Corban is a clear illustration; its omission tips the balance of the pericope even more toward "defilement." (So, okay, I will quibble a bit: even though Corban is an alien custom to us, Jesus's illustration still works to raise the prophetic issue about idolatrous priorities... even for us.)
With the second omission (verses 17-20), we miss hearing that the final teaching is directed only to the disciples. Because the teaching to the disciples in verse 20 echoes and then in verse 21 amplifies what Jesus has already said to the crowd, the edited pericope flows seamlessly. The effect though is to make the pericope sound more like a wisdom saying than discipleship teaching.
As I've already suggested, this pericope poses a challenge for contemporary Christian preaching because so much of its rhetoric deals with an issue we don't feel in our bones. We can respect others' religious concern for ritual purity and defilement, but from our post-resurrection, post-Paul (not to mention fully American) perspective of freedom, it feels quaint and archaic. Conversely, there are rich issues in the pericope that do resonate with us--mercy being eclipsed by narrow focus on "church craft"; human tradition elevated over God's mission; and the naked truth about our evil hearts--but they are cloaked by all the pericope's verbiage about defilement.
With all that said, when I'm sitting in the pew on August 30, I hope to hear you lead us into this text in one of these two ways:
1. Help us to understand which church traditions and practices we so busy ourselves with that we lose touch with God's heart. To what would Jesus point and say, "You abandon the commandment of God and hold to human tradition?" The success of this approach will rest on the preacher's skill in handling this subtle nuance: the practice in question should be one we cling to not out of mere stubbornness, taste, or inertia but out of a desire to stay dutiful or "pure"--or perhaps we might say, "right with God." (Maybe "right with God" language is our rough analogy to "ritual purity.")
The payoff of achieving this subtlety will be a sermon that leads us to a deeper understanding of idolatry and self-righteousness. When the core commitments of divine law (mercy, justice, and even care for parents) are eclipsed by human teachings and practices, it means that we have chosen to determine for our own selves what makes us righteous (self-righteousness), so washing the fruit we buy becomes more important than giving fruit to the hungry. A preacher might profitably (and prophetably) help us to know when we have crossed the line from "mere hypocrisy" (cynically ignoring the practice of God's law even as we proclaim it) into full-fledged idolatry and self righteousness (cobbling a twisted version of divine law that suits our needs and desires).
We have to take the text at face value and suppose that the Pharisees and scribes are guilty of hypocrisy as charged. But an imaginative gloss on the story might invent a religious leader, quick on the uptake, who replies to Jesus, "Yes, Jesus, of course we know that it is our actions that defile us. And we know that evil actions are motivated by unclean hearts. But these rituals that seem so arbitrary are part of a practice of community discipline that we believe helps to form and guard our hearts. It's all very well and good for you to say, 'You're free! Eat whatever you want.' But St. Paul himself will someday say in Romans 14, 'If your brother or sister is being injured by what you eat, you are no longer walking in love.'"
2. Help us to push back (reverently) against Jesus's digestive dualism by both complexifying our notion of ingestion and contemporizing our idea of defilement. Jesus says "there is nothing outside a person that by going in can defile" but no recovering addict whom I know would say that ingestion and the formation of the heart are unrelated. It is a cliché and also true to say that in our culture we use food and mood-altering substances to self-medicate for spiritual issues--emptiness, pain, boredom, anxiety. The issue here is not ritual defilement but rather such effects as our guilt or shame at our own indulgences and our powerlessness over them (our self-defilement, if you will).
This next homiletical prompt will require a "spiritualization" of the pericope that may not be comfortable for all. Even if our "spiritual hearts" are immune to contamination from the preservatives in Pringles; our souls are indiscriminate and relentless consumers, imprinted and formed by everything to which they are exposed.
In this respect many things "outside a person" can defile: the banality of the twenty four hour news cycle or Gilligan's Island reruns; the depersonalization of internet pornography or endless new games of WordTwist; the romanticizing and sentimentalizing of human life experience for the manipulation of economies. These things do not move cleanly through our various tracts and systems and out into the sewer. Rather they flatten us, desensitize and anesthetize us, they skew our perspective and sense of proportion and thereby deform the very "hearts" from which evil intentions come.
My plea earlier was that you begin your sermon prep by asking, "What are God's specific gracious remedies to our defilement?" Perhaps a starting place is to rejoice that God creates in us new hearts and right spirits within us.