Do Deuteronomy 6:4 and Psalms 110:1 teach the trinitarian plurality of God? Let's find out.
By rendering Psalms 110:1 as, ". . . the Lord said to my Lord . . ." Christians argue that Jesus is greater than David and is not only the Messiah but is part of a trinitarian godhead as well (see Matthew 22:42-45, Mark 12:35-37, Luke 20:41-44, Acts 2:34-36, Hebrews 1:13). Yet, a careful examination finds their hypothesis to be totally without merit.
Since le-David, in verse 1, does not always mean "written by David," but sometimes "concerning David" or "in the style of David," it cannot be said with certainty that the preposition le, often translated "of," actually indicates "composed by David." Further investigation is necessary in order to understand its meaning as governed by the context of this psalm.
Let us examine Psalm 72. It was written by David "for," or "concerning," Solomon (cf. verses 1 and 20), yet the Hebrew contains an introductory phrase similar to the one found in Psalm 110. The introductory statement, li- S'hlomo, stresses that the psalm is "concerning" Solomon rather than that it is by Solomon. Even more significant is 2 Samuel 22:51 and Psalms 144:10, where David speaks of himself in the third person. Accordingly, there is every indication that the proper translation of Psalms 110:1 is: "A Psalm concerning David. HaShem says to my master ['adoni]: 'Sit at My right hand, until I make your enemies your footstool.'" David is writing this psalm from the perspective of the individual who is going to recite it. From this perspective, David, as king, is appropriately referred to as "my master." The claim that David is actually (or also) referring to Jesus by the phrase "my master" is not supported by the text.
The privilege of sitting at the right hand is a mark of distinction (1 Kings 2:19). When God invites David to "sit at My right hand," it is to show the privileged position enjoyed by David in his relationship with God. It is not to be taken as literally indicating sitting at God's right hand. The terminology "right hand" is here used as an expression of God's favoritism toward David.
From a Christian perspective: Does the name of God (HaShem), translated as "the Lord" in many English versions of Psalms 110:1, refer to "God the Father" or to "God the Son" or does it refer to all three members of the Trinity? Christians are divided on the Answer.
Concerning the word 'Elohaynu ("our God"), which appears in the Shema, "Hear, O Israel, the Lord [HaShem] our God, the Lord [HaShem] is One ['Echad]" (Deuteronomy 6:4), most Christians maintain that it is plural and should be understood in its literal sense as "our Gods," but in the sense of a "triunity." For this reason, they often interpret the verse as: "Hear, O Israel, the Lord our Gods, the Lord is a compound unity."
From this Christian explanation of the Shema, it follows that "the Lord" (HaShem) could not refer to either "God the Father" or "God the Son" alone, but must refer to all three members of the "triunity" as a whole. This being the case, how is it possible for Christians to maintain that the phrase "to my Lord" (as commonly translated in Christian Bibles) refers to Jesus? If "my Lord" refers to the second member of the supposed "triunity," Jesus, then who is the first "Lord" mentioned in the verse? If "the Lord" (HaShem) in the Shema is a "triunity" united in the divine name, that is, "the Lord is our Gods," the first "Lord" in Psalms 110:1 must also refer to the united "triunity." If this is so, then the phrase "to my Lord" automatically excludes Jesus, who allegedly is already included in the first part of the verse, "the Lord."
Furthermore, if the second "Lord," supposedly Jesus, is sitting NEXT to the first "Lord," the triune godhead or two-thirds of it, or any aggregate of it, he cannot be part of it. That which exists outside of God cannot be God.