But in my lesson preparation I'm reading Luz's massively detailed Hermeneia commentary, and I can't help but notice that some anachronistic concepts seem to sustain his (and many others') interpretation. What's worse, the assumption I have in mind is never explicitly discussed, which makes it all the more problematic! A commentary might not be the place to engage such things, but given their effect on our exegetical work I think they deserve some attention.
Specifically, I have in mind the assumption that Christianity and Judaism were, already in the first century CE, separate and distinguishable entities. The very opening of Luz's interpretative comments on Matt 5.17–20 he employs this assumption and allows it to drive his reading of this very important passage:
By placing these verses at the beginning of the main part of the Sermon on the Mount before the antitheses, Matthew makes clear that they are fundamentally important for him. At issue here is his relationship to the Mosaic Law and thus to Judaism. (Luz, Matthew 1-7, 213; my emphasis)
I'm not at all convinced that the issue here is "his relationship to the Mosaic Law"; rather, I think the point is rather clearly the interpretation of Torah and, as emphasized in v 20, the proper observance of Torah. Of course, if Jesus ever did say anything like 5.17–20, we would struggle to explain why Jesus would have to explain to his fellow Jews that he recognized the authority of the Mosaic covenant. Certainly at the front-end of Jesus' ministry, as Matthew has placed the Sermon, we have yet to see anything that would make us think Jesus set out to live free of Torah's strictures and ordinances (Moses might have preferred "blessings and curses"). So why should Jesus have to explain his "relationship to Judaism"?
But Luz doubts the authenticity of this passage; at the very least he reads 5.17–20 as speaking "directly to the church" (213). And if Jesus, in Matthew's gospel, has yet to fall foul of Torah, the church, by the time Matthew was written, certainly had. And given the debates attested earlier in Paul's letters and later in Luke-Acts, Matthew writes to a church whose relationship to Judaism was open to question and required some explanation.
The problem with all of this, I think, is that we cannot invoke the neat distinctions between Judaism and Christianity to structure our interpretation of the NT texts. Daniel Boyarin (Dying for God , Border Lines ), among others, has demonstrated the messy interactions between the expressions of Judaism that became Rabbinic Judaism and Christianity. We cannot helpfully think of these as two distinct phenomena at any point in the first century CE; even thinking in terms of two phenomena sends us off on the wrong path.
I haven't yet read Luz's interpretation of Matt 5.17–20; I stopped to get these ideas out of my head. But I'm suspicious that he began with what I think is a very problematic assumption: that Matthew had to clarify "his relationship to the Mosaic Law and thus to Judaism." The text offers significantly more potential, I think, if we begin instead by understanding Matthew competing with other Jews (who may or may not have identified Jesus of Nazareth as a prophet or even Israel's messiah) over how to properly understand and practice Torah.