Drawing from the only other mention of Melchizedek besides Psalm 110:4, he describes his meeting with Abraham when Abraham was returning from the slaughter of the kings in Genesis 14. Hebrews is only concerned with Melchizedek insofar as much as [sic] he relates to Christ, and receives attention simply because his priestly office and this meeting with Abraham supply evidence for the writer's main thesis, viz., that there is a new priesthood that is superior to the old. (135–136; my emphasis)
Forget the grammatical problem in the middle of the quote; any work of this length and sophistication will suffer a few problems like this. Forget even the non sequitur at this quote's beginning, in which Joslin refers to Genesis 14 as "the only other mention of Melchizedek besides Psalm 110:4." Nothing about Genesis 14 is "the only other mention"; Genesis 14 provides the account of Melchizedek and Abraham, and Psa 110.4 is "the only other mention" of Melchizedek. But Joslin misses this because he isn't concerned with Melchizedek; he's only concerned about priesthood, and Melchizedek is a label that simply means "not-Levitical" (see 135, n. 7). The text might as well have said Christ is a priest forever according to the order of Gidget, and only the consonants מלכי־צדך [mlky-ṣdk] in Psa 110.4 prevented him from doing so.
But I'm struggling with the point that Joslin reads into Hebrews 7. Given the text's logic—that Levi was still in Abraham's loins when Abraham offered his tithe to Melchizedek and so Levi offered tithes to Melchizedek—I don't see how the writer's point could possibly be that Christ belongs to a new priesthood, and that this new priesthood is superior to the old one. If anything, it seems to me that Hebrews places the Levitical priesthood in the category new; Christ's priesthood, then, being according to the order of Melchizedek, is both older and, therefore, superior.
Am I missing something here?!