Unitarians are very fond of the Gospel account of the Lord Jesus where He and a scribe discuss the greatest commandment. They point to the fact that Christ states that the greatest commandment is the Shema of Deuteronomy 6:4, and then point to the fact that Christ states that the scribe answered wisely. What they fail to do, however, is actually read the text, for in this passage the emphasis on the Trinity is unmistakable. To begin with, we need to put aside the presupposition of the Unitarian, viz. that the “Oneness” of God refers to both His Being and His Personhood. Rather than beginning with the very point that is under consideration, the Unitarian must look at what Scripture clearly presents to the reader.
We need to note that Christ is addressing His enemies, men who are trying to trap Him so that they can accuse Him of sin and, thereby, put Him to death according to their customs. We see this in Matt 22:34-35, where the Pharisees and Scribes huddle together and a Scribe/Lawyer asks Him the question: “Teacher, which is the greatest commandment in the Law?” The Lord Jesus responds by quoting the Shema, which is given to us in Mark 12:29: “Jesus answered, ‘The most important is, “Hear, O Israel: The Lord our God, the Lord is one.”’” The Scribe agrees with Christ, yes, and Christ sees that the scribe answered wisely, yes, but the remainder of the text dispels the Unitarian myth of Christ’s approbation of the scribe’s theology when it goes on to record (i.)Christ’s interpretation of Psalm 110:1 and (ii.)Christ’s stinging rebuke of the Scribes and the Pharisees en masse as recorded in brief in Mark 12:38 and in greater detail in Matthew 23:1-36. Any attempts to see in the Lord’s words anything close to approval for the Scribe’s theology is really missing the point, for as soon as He states that the scribe is not far from the Kingdom of Heaven, the Lord Jesus goes on to speak judgment against the very religious group that man belonged to, because of their heresy and heteropraxis.
How do we know this? By the simple fact that the emphasis given in Mark 12:29-32 is on the Oneness of the “Lord” of God’s people who is alone to be worshiped. And just a few verses later, Christ asks the question: “How can the scribes say that the Christ is the Son of David? David himself, in the Holy Spirit, declared, ‘The Lord said to my Lord…’David himself calls Him Lord. So how is He his Son?” Christ’s point here is that He is David’s Lord. The scribes, therefore, are not realizing that by not honoring the Son as they claim to honor the Father, they are breaking the first and greatest commandment. “The Lord” is the One to be worshiped, even as David looked forward to Christ and called Him his Lord. Christ, therefore, is agreeing with the scribe in one sense: He is approving the prima facie understanding of the text that the scribe, but by designating Himself as Lord of David, He is directly associating Himself with the Shema, the One Lord.
So the pertinent question for the Unitarian, as it was for the scribes and Pharisees is: “If there is One Lord, then why does David call Christ his Lord?” If it is the case that there is only One Lord, then how is Christ also Lord? Is He contradicting Himself? Is He stating that it is the case that there are two “Lords” who preside over Israel as God and King? This would be a contradiction. Is He saying that He is a lesser “Lord” than the One Lord found in the Shema? Then why is He given all authority? How does He exercise the Divine prerogative of Judging all mankind? Christ is Lord, therefore, equally as the Father is Lord, and although they are distinct Persons, they are One Lord. The Shema itself hints at this plurality within the Divine Essence by stating (i.)the Lord, (ii.)our God, (iii.) the Lord is One. Three appellations are given, but One Being, Supreme and worthy of all worship is signified.
While in Hebrew there is a difference between the two words for “Lord,” in the Greek the term is the same. So there is no argument regarding the difference between the tetragrammaton and ʾādôn, seeing as both may are replaced by the Greek kurios. As the Father, therefore, is kurios, so the Son is kurios – and yet there is but One kurios. The Unitarian cannot explain why this is the case, but Trinitarians can. The One True Lord of Israel is Father and Son, and the Holy Spirit is mentioned here as well.
 The two words used respectively for “The LORD” and “my Lord” in Psalm 110:1 are יהוה (YHWH, or Yahweh) and אָדוֹן (ʾādôn).