Joslin provides an exegetical discussion of Heb 8.1–13 in chapter five. Hebrews 8, of course, has the lengthy reference to Jer 31.31–34 [LXX = 38.31–34] in which YHWH promises a "new covenant" with the houses of Judah and Israel. Throughout Joslin's book this "new covenant" self-evidently implies the passing of the old, Mosaic covenant. The reference to Israel and Judah, then, become somewhat of a problem. Joslin writes,
Though Jeremiah specifically addresses Israel and Judah, it becomes clear that Hebrews sees the prophecy's fulfillment in the eschatological days of the present time, and that it is applicable to more than simply the Jewish nation, though clearly they are not excluded. (186; my emphasis)
The problem I have, I think, begins with the two clauses that Joslin introduces with the concessive conjunction "though." First, Joslin has to find a way to diminish the Jeremianic reference to Israel and Judah. I don't necessarily fault Joslin here; he's writing about Hebrews, not Jeremiah. If Hebrews diminishes the explicitly Israelite/Judahite focus of Jer 31.31–34, Joslin would need to do so as well. I'm not convinced that Hebrews does this, however, but that's another (though related) subject. In fact, now that I look back at Joslin's quote, this seems to be precisely his point: Jeremiah focuses on Israel and Judah; Hebrews, not so much.
But, and second, the second though-phrase has to back-pedal and reserve a place for Israel and Judah within the Jeremianic new covenant. At this point, then, either the author of Hebrews has misread Jeremiah, or Joslin has misread Hebrews. And it isn't simply deference for Hebrews's privileged place within the canon that makes me prefer the latter option. Hebrews invokes a world determined and defined by Hebrew biblical tradition, by the story of Israel's God and his relationship with his covenant people. Indeed, Hebrews takes up and manipulates those traditions and that story in order to achieve certain ends in its own present.
But one thing Hebrews—and the rest of the New Testament, it seems to me—does not do is "reserve" a place for Israel. The texts comprising the New Testament do not regard Israel (and the Jews enveloped within that label) as ancillary to God. Those texts, in different ways but all together, begin with a notion of God's election of Israel and proceed (again, in different ways but all together) to extend that notion to the nations beyond Israel. Any argument that has to include a caveat such as, "though clearly they are not excluded," is already moving in the wrong direction.
Whence comes this analytical anomaly? The self-evident split between Christianity and Judaism in our own world, I think, has distorted our analyses of the NT texts, of Christian origins, and of late Second Temple Judaism in general. Given that the NT texts are Christian texts, and given that Christianity ≠ Judaism, critical analyses have to explain the role Jews play in the symbolic universe(s) built and inhabited by Jesus' followers. As a result, scholarly discourse on Christian origins resembles a horse being led around by its cart, and exegetical work (such as Joslin's) guided by such a wayward horse is equally lost.
To be clear, eventually Christianity would become something other than Judaism, a fact that present religio-political realities make obvious. And even in antiquity, theologians and bishops comfortable wearing the label Christian but who refused the label Jew would have to wrestle with their strange patterns of identification. But the texts of the NT—and the authors thereof—don't seem to have made this distinction. The point in Heb 8.1–13 is clearly not that the Jews were clearly not excluded from the new covenant. This was never in doubt!