“Yahweh has sworn.
He will not change his mind,
‘You are a priest forever
According to the order of Melchizedek.’”
Yahweh then somehow makes David a priest also. The allusion here is to Melchizedeck, the ancient priest and king at the time of Abraham. This King Melchizedek of Salem, as found in Genesis, chapter 14, offered a sacrifice of bread and wine for Abraham. Here there is an order of Melchizedek, which will become an important Christian emphasis within the Roman Catholic concept of the priesthood. This same phrase was repeated in the New Testament Epistle to the Hebrews, chapter 5.
Eugene Finnegan Bible Project 