Psalm 110 (Douay-Rheims Psalm 109) begins with a riddle Jesus Himself posed to the Pharisees: 'The Lord said TO MY LORD: Sit thou at my right hand' — if the Messiah is merely David's son, 'how then doth David in spirit call him Lord?' (Matthew 22:41–46; Psalm 110:1). It is the Old Testament verse most quoted in the New. The psalm then swears an oath found nowhere else: 'The Lord hath sworn, and he will not repent: Thou art a PRIEST FOR EVER according to THE ORDER OF MELCHISEDECH' (110:4).
Melchisedech appears for three verses in Genesis: 'king of Salem' — the future Jerusalem — and 'priest of the most high God,' who 'brought forth BREAD AND WINE' and blessed Abraham (Genesis 14:18–20). A priest-king, before Levi existed, without recorded genealogy, whose offering was bread and wine: the Epistle to the Hebrews builds its entire theology of Christ's priesthood on the figure (Hebrews 5–7), arguing that a priesthood sworn 'for ever,' outside the law of descent, has superseded the Levitical line.
The Church reads the fulfillment as sealed in the upper room and on the Cross — the true Priest-King of Salem offering Himself under the signs Melchisedech carried, bread and wine — and hears the psalm at every ordination. The Roman Canon still names him at the altar: 'the offering of thy high priest Melchisedech.'