Melchizedek serves an important theological role in the book of Hebrews. The author of Hebrews interprets Psalm 110 to speak of Jesus' priestly order (Melchizedek), thus resolving the potential problem that Jesus might have been suspected to be an illegitimate priest since he was not named after the order of Aaron.
As I understand it in Hebrews 7, he makes the argument that because because Abraham tithed to Melchizedek, it demonstrates that Abraham was blessed in some legitimate priestly fashion by Melchizedek, and that the one who blesses is superior to the one who receives the blessing, therefore Melchizedek is even superior to Abraham, which makes it quite fitting that Jesus would be after this priestly order, not the priestly order with descendants of Aaron who are limited by their own death. One might venture to guess from this theological interpretation that Melchizedek might have even been a type of theophany of Jesus, or if not, that his priesthood was special, unparalleled, and arcane.
However, the Anchor Yale Bible Commentary on the passage in Genesis 14:1-24 in dealing with the question of authenticity notes (p. 109):
Finally, the notice about Melchizedek merits a measure of confidence in its own right. He invokes an authentic Canaanite deity (see NOTE) as good Canaanite priest would be expected to do. Abraham, on the other hand, refers to Yahweh, using the Canaanite name or names in suitable apposition, which is not less appropriate in his particular case.
The NOTE referenced mentions:
El-Elyon. Both elements ('el and 'elyon) occur as names of specific deities, the first in Ugaritic and the second in Phoenician; the Aram. inscription from Sujin combines the two into a compound. Though appellatives at first ("god" and "supreme" respectively), both are attested also as personal synonym for Elohim; and 'elyon occurs either separately (Isa xiv 14; Ps ix 3), or as a divine epithet (Pss vii 18, xlvii 3, lvii 3, lxxviii 56). But these are relatively late passages which conceivably could hark back to instances before us. The question, then, is how to interpret the latter.
Now that this chapter is amply attested as a source unto itself, it is not only unnecessary but fallacious to harmonize its contents with other portions of the OT. As a Canaanite priest, Melchizedek would invoke his deity or deities by name; and this is what the above translation sought to reproduce. Abraham would just as naturally turn to Yahweh, especially in an oath.
While Speiser (the author of AYB Commentary on Genesis) argues that it's fallacious to harmonize El-Elyon, it seems to be attempting to prove too much from Melchizedek's choice of deity name he invokes. Melchizedek appears upon the end of a bloody struggle between pagan kings, but was not himself involved in the battles. His words impose divine significance upon Abraham's victory, which make little sense if Abraham does not believe in his god. Further, it would seem to undermine the general message of the Pentateuch. Why would the author of the Pentateuch who bundles law about the Levitical priesthood receiving tithes, and the Canaanite deities being abominations, include Abraham acknowledging the legitimacy of the pagan Canaanite priesthood of Melchizedek by paying him a tithe?
What's Speiser's argument here? Is it more than a simple reaffirmation of the source hypothesis?