The Lord Said Unto My Lord
Psalm 110, David's Lord and the eternal priest, as the early church read it
No Old Testament verse is quoted more often in the New than Psalm 110:1, "The LORD said unto my Lord, Sit thou at my right hand." Jesus Himself used it to silence the Pharisees: if David calls the Messiah "my Lord," how is the Messiah merely David's son (Matthew 22:41-46)? Peter preached it at Pentecost, and Hebrews builds its whole doctrine of Christ's priesthood on its fourth verse, "Thou art a priest for ever after the order of Melchizedek." Justin Martyr, debating the Jewish teacher Trypho around AD 155, presses both points: the One seated at God's right hand is no mere man, and the everlasting priest can only be Jesus. His words below, with a plain restatement.
The Father's words are verbatim and attributed (Justin Martyr, Dialogue with Trypho, in the Ante-Nicene Fathers, public domain; selected from the running prose, footnote apparatus omitted). The box marked "In plain terms" is our own restatement, never the Father's words.
Psalm 110:1 · KJVThe LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.
"I will mention to you other words also spoken by the blessed David, from which you will perceive that the Lord is called the Christ by the Holy Spirit of prophecy; and that the Lord, the Father of all, has brought Him again from the earth, setting Him at His own right hand, until He makes His enemies His footstool; which indeed happens from the time that our Lord Jesus Christ ascended to heaven, after He rose again from the dead."
Justin MartyrThe force of the verse is in the two "Lords." David, the king, the highest man in Israel, calls someone "my Lord," and that Lord is invited by God Himself to sit at His right hand. As Jesus argued, no mere descendant of David fits; the Messiah is David's Lord, above him. Justin reads it exactly as the church did: this is Christ, "brought up from the earth" in the resurrection and "set at God's right hand" in the ascension, now reigning until every enemy is made His footstool.
Psalm 110:4 · KJVThe LORD hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek.
"'The Lord hath sworn, and will not repent,' it is said; and, 'Thou art a priest for ever, after the order of Melchizedek'… Not even you will venture to object that Hezekiah was either a priest, or is the everlasting priest of God; but that this is spoken of our Jesus, these expressions show… with an oath God has shown Him to be the High Priest after the order of Melchizedek… God has shown that His everlasting Priest, called also by the Holy Spirit Lord, would be Priest of those in uncircumcision."
Justin MartyrThe fourth verse names something no Israelite king ever was: a priest, and an everlasting one, "after the order of Melchizedek," the mysterious priest-king of Genesis 14 who blessed Abraham, with no recorded beginning or end. Justin's point is simple: this priesthood cannot belong to Hezekiah or any mere man; it belongs to Jesus, sworn into office by God's own oath, a priest forever. And because Melchizedek served before circumcision and the law, Christ's priesthood reaches beyond Israel to "those in uncircumcision," the nations. Hebrews would unfold this at length: one Priest, one offering, once for all, who "ever liveth to make intercession."
Where this stands among the traditions
This is shared ground, and the New Testament stakes it out plainly: Jesus presses Psalm 110:1 on the Pharisees, Peter cites it at Pentecost (Acts 2:34-35), and Hebrews quotes both verses to prove Christ's exaltation and His eternal priesthood. So Catholic, Orthodox, and Protestant confess together that the psalm sings of the risen, ascended, reigning Christ, David's Lord and our great High Priest. The divergence is the older one Justin met directly: the Jewish reading that referred the psalm to a merely human king, Hezekiah or another, which Justin answers by noting that no son of David was ever "a priest for ever." A modern critical reading similarly treats it as a court psalm for an Israelite monarch, draining away the "David's Lord" force that Jesus Himself drew out. The drift to resist, then, is twofold: emptying "the LORD said unto my Lord" of the divine dignity the Lord Jesus claimed from it, and losing sight of His once-for-all Melchizedek priesthood, the ground of our access to God, whether by forgetting Christ's priestly mediation altogether or by multiplying mediators He has already surpassed. (See the Son is God and the triumph of the cross.)
Patristic text from Justin Martyr, Dialogue with Trypho (ch. 32-33), in the Ante-Nicene Fathers (public domain), selected from the running prose with footnote apparatus omitted; nothing added or paraphrased within the quotation marks. Scripture in the King James Version; the plain-language lines are our own restatement. This passage in the Study Bible; Psalm 110 at BibleHub.