Paul Tillich, commenting on the Apostle Paul's assertion that the gospel is a stumbling block, once said that the danger is stumbling over the wrong thing.
There is something similar occurring in today's reading, so we'll want to take our interpretive steps with care.
It's difficult to avoid interpreting the parable in straightforward, even simplistic terms, in part because the dramatic action of this parable is so very predictable even to those with only limited knowledge of the story of Jesus' life. Knowing that Pharisees are regularly cast in the gospels as Jesus' opposition, we all too easily judge the Pharisee to be a self-righteous hypocrite and assume that the moral of this story is to be humble. There is good reason for this straightforward interpretation, as Luke seems to frame the parable in just these terms. The difficulty with such an interpretive tact, however, is that we might as well end up preaching, "Lord, we thank you that we are not like other people: hypocrites, overly pious, self righteous, or even like that Pharisee. We come to church each week, listen attentively to Scripture, and we have learned that we should always be humble."
In order to avoid the kind of self-congratulatory reading of the parable that the parable itself would seem to condemn, it may help to note that, in fact, everything the Pharisee says is true. He has set himself apart from others by his faithful adherence to the law. He is, by the standards both Luke and Jesus seem to employ, righteous (see Luke 15:7). So before we judge him too quickly, we might reframe his prayer slightly and wonder if we have uttered it ourselves. Maybe we haven't said, "Lord, I thank you that I am not like other people...", but what about, on seeing someone down on his luck, "There but for the grace of God go I"? It isn't that the Pharisee is speaking falsely, but rather that the Pharisee misses the true nature of his blessing. As Luke states in his introductory sentence, he has trusted in himself. His prayer of gratitude may be spoken to the Lord, but it is really about himself. He locates his righteousness entirely in his own actions and being.
The tax collector, on the other hand, knows that he possesses no means by which to claim righteousness. He has done nothing of merit; indeed, he has done much to offend the law of Israel. For this reason he stands back, hardly daring to approach the Temple, and throws himself on the mercy of the Lord.
Here is the essential contrast. One makes a claim to righteousness based on his own accomplishments, while the other relies entirely upon the Lord's benevolence. Rather than be grateful for his blessings, the Pharisee appears smug to the point of despising others. In his mind there are two kinds of people: the righteous and the immoral, and he is grateful that he has placed himself among the righteous. The tax collector, on the other hand, isn't so much humble as desperate. He is too overwhelmed by his plight to take time to divide humanity into sides. All he recognizes as he stands near the Temple is his own great need. He therefore stakes his hopes and claims not on anything he has done or deserved but entirely on the mercy of God.
I don't think it's an accident that this exchange takes place at the Temple. On the grounds of the Temple, you were always intimately aware of who you were, of what status you had, of what you could expect from God. There were, at the Temple, "insiders" and "outsiders," and according to these rules there was no question of where the Pharisee and tax collector stood. But when Jesus dies all this changes. As the gospels report, the curtain in the Temple is torn in two (Luke 23:45), symbolically erasing all divisions of humanity before God. That act is prefigured here, as God justifies not the one favored by Temple law, but rather the one standing outside the Temple gate, and aware only of his utter need.
This is what makes this parable so hard to preach. Indeed, what makes this parable a trap. For as soon as we fall prey to the temptation to divide humanity into any kind of groups, we have aligned ourselves squarely with the Pharisee. Whether our division is between righteous and sinners, as with the Pharisee, or even between the self-righteous and the humble, as with Luke, we are doomed. Anytime you draw a line between who's "in" and who's "out," this parable asserts, you will find God on the other side. Read this way, the parable ultimately escapes even its narrative setting and reveals that it is not about self-righteousness and humility any more than it is about a pious Pharisee and desperate tax collector. Rather, this parable is about God: God who alone can judge the human heart; God who determines to justify the ungodly.
At the end of this story, the Pharisee will leave the Temple and return to his home righteous. This hasn't changed; he was righteous when he came up and righteous as he goes back down. The tax collector, however, will leave the Temple and go back down to his home justified, that is, accounted righteous by the Holy One of Israel. How has this happened? The tax collector makes neither sacrifice nor restitution. On what basis, then, is he named as righteous? On the basis of God's divine fiat and ordinance! This parable is therefore preached well only to the degree that each time we try to interpret it we find ourselves, yet again, with nothing to claim but our dependence on God's mercy. When this happens and we forget if only for a moment our human-constructed divisions and stand before God aware only of our need, then we, too, are justified by the God of Jesus and invited to return to our homes in mercy, grace, and gratitude.