In v. 4 the ‘lord’ is given a second
oracle under Yahweh’s oath: he is a priest for ever after the order of
Melchizedek. It is widely noted that in early times David acts as both king and
priest: he wears the priestly ephod, conducts the ark, offers the sacrifices,
makes his sons priests, and so on. Only in later times does it come to be felt
that sacrifice is for priests only, and Uzziah is stricken with leprosy when he
offers incense in 2 Chronicles 26. Priests were under special divine
protection, so the king was happy to have this additional guarantee. Further,
appeal is made to Gen. 14:18–22, where Melchizedek was both king of Salem and
also priest of אל עליון.
The king of Jerusalem in Joshua 10 is called Adoni-zedek, and this led to the
conclusion that the god worshipped in Canaanite Jebus was אל עליון,
with an association to a god Zedek; and that Zadok had been priest-king of the
place. The oracle would then be divine confirmation that David has succeeded to
the joint authority in perpetuity. (Michael D. Goulder, The Psalms and the Return:
Book V, Psalms 107-150 [Journal for the Study of the Old Testament
Supplement Series 258; Sheffield: Sheffield Academic Press, 1998], 143-44)
