Charles Ellicott Commentary Psalms 110:4

"Jehovah hath sworn, and will not repent: Thou art a priest for ever After the order of Melchizedek." — Psalms 110:4 (ASV)

After the order of Melchizedek. — This follows the Septuagint and Vulgate. Better, after the manner of, since the psalmist could have had no intention of contrasting this priesthood with that of Aaron, as there naturally was when the Aaronic order had come to an end or was visibly doomed to extinction.

The previous history of Israel itself offered no example of the formal union of kingly and priestly offices in one person. It first appears in idea in Zechariah 6:12-13; in actual fact in the pontificate of Jonathan . It is true that the royal and priestly functions were sometimes united, especially in the case of David, and in 2 Samuel 8:18, David’s sons are called “priests” (in the English version, “chief rulers; ” margin, or princes). It was therefore necessary to go back to Melchizedek, in whom history recognized this sanctioned and formal union (Genesis 14:18). For the various points brought out in the Epistle to the Hebrews 6:7, see New Testament Commentary.