Psalms — Chapter 109
DIXIT DOMINUS DOMINO MEO: SEDE A DEXTRIS MEIS. Lapide: Psalm 109 is the most frequently quoted Old Testament verse in the New Testament (Mt. 22:44; Mk. 12:36; Lk. 20:42-43; Acts 2:34-35; Heb. 1:13, 5:6, 7:17,21, 10:13; 1 Cor. 15:25; Eph. 1:20; Col. 3:1; 1 Pet. 3:22 and more). "The Lord said to my Lord" — David, inspired by the Holy Spirit, calls the Messiah "my Lord" (Domino meo). Christ uses this against the Pharisees (Mt. 22:41-46): if the Messiah is merely David's physical descendant, why does David call him "Lord"? They cannot answer. Lapide: the first "Dominus" = the Father; the second "Domino meo" = the Son, who is David's Lord because He is God. This verse establishes the Divinity of the Messiah from the mouth of David himself. "Sede a dextris meis" — the right hand = equality of nature, glory, and power with the Father. The Ascension and Session at the right hand fulfilled (Acts 2:33).
Verse 4
Tu es sacerdos in aeternum secundum ordinem Melchisedech
Lapide treats this as one of the most theologically dense verses in Scripture, cited in Hebrews 5:6, 7:17, 7:21. The priesthood of Melchisedech — offering bread and wine, without genealogy, without beginning or end of days — prefigures Christ's eternal priesthood precisely in the Eucharist: bread and wine offered on the altar is Christ's own sacrifice perpetuated. Lapide uses this verse as one of the proof texts for the sacrificial character of the Mass (as defined at Trent, Session 22). The 'in aeternum' (forever) distinguishes Christ's priesthood from the Aaronic, which was temporal and shadowy. He cites Chrysostom, Ambrose, and Theodoret on why the comparison to Melchisedech rather than Aaron establishes the universal and eternal character of the New Covenant priesthood.