Deuteronomy 6:4 is a famous verse: “Hear, O Israel, The Lord our God is one Lord.” (LXX Bible). It is used not only by Christians, but also Jews and various other sects to prove that the scriptures teach monotheism, the belief that there is only one God.
This is of course true; but it is also noteworthy that this passage of scripture tells us that God is one and does not include an explanation of what is meant by this in relation to the Trinity. Revelation was progressive, and at this point in history, not as much detail had been revealed about the Trinity in the scriptures. That said, this verse is not primarily intended to teach us about the doctrine of the Trinity; it is a blanket statement of monotheism.
As previously discussed in The Priority of the New Testament in Trinitarian Doctrine, we must read less clear passages of scripture with the aid of those which are more clear. When a given passage of scripture reveals that there is only one God, and does not speak in further detail to how this fits with the doctrine of the Trinity, our first response should be to seek clarification on this topic from other passages of scripture that speak to this point. It is unwise to simply jump to trying to invent our own custom interpretation of the passage without examining it in light of other related passages of scripture.
When we look at New Testament passages related to Deut. 6:4, we find several. Firstly let us note that it is quoted in Mark 12:28-34:
“Then one of the scribes came, and having heard them reasoning together, perceiving that He had answered them well, asked Him, “Which is the first commandment of all?”
29 Jesus answered him, “The first of all the commandments is: ‘Hear, O Israel, the Lord our God, the Lord is one. 30 And you shall love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength.’ This is the first commandment. 31 And the second, like it, is this: ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.”
32 So the scribe said to Him, “Well said, Teacher. You have spoken the truth, for there is one God, and there is no other but He. 33 And to love Him with all the heart, with all the understanding, with all the soul, and with all the strength, and to love one’s neighbor as oneself, is more than all the whole burnt offerings and sacrifices.”
34 Now when Jesus saw that he answered wisely, He said to him, “You are not far from the kingdom of God.”
But after that no one dared question Him.” (NKJV)
This whole exchange is admittedly vague enough to say that it does not speak with certainty as to who the person Deuteronomy 6:4 refers to is, but it is noteworthy at the very least that the Lord gives no indication whatsoever that he views that verse as referring to Himself. This is significant, because if the Trinity in totality were being referenced there, as semi-modalists suggest, then it would refer to Christ, along with the Father and the Spirit. The lack of any indication this is the case leaves no support for interpreting Deuteronomy 6:4 as referring to the whole Trinity in this passage.
Let us then examine other passages which could be considered parallel in the New Testament, inasmuch as they also speak of the fact that there is only one God:
“There is one body and one Spirit, just as also you were called in one hope of your calling; 5 one Lord, one faith, one baptism, 6 one God and Father of all who is over all and through all and in all.” (Ephesians 4:4-5 NAS)
“This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.” (John 17:3 NAS)
“yet for us there is but one God, the Father, from whom are all things and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him.” (1 Corinthians 8:6 NAS)
Unlike Deuteronomy 6:4, these passages not only state clearly that there is only one God, but also explicit identify Who is referred to by that title: the person of the Father in particular. Theses passages, unlike Deuteronomy 6:4 go beyond merely affirming monotheism to include detail on how these statements fit with the doctrine of the Trinity, by identifying the one God as the first person of the Trinity.
If then we are willing to read the less-clear passage, Deuteronomy 6:4, with the assistance of these more-clear passages, we will be forced to admit that the most reasonable interpretation of Who Deuteronomy 6:4 is referring to is the person of the Father alone. This is to read the less-clear passage in light of the more-clear, and to read both Testaments in tandem with each other, assuming that when scripture speaks of there being one God in the Old Testament it means the same thing, and refers to the same person, as the the New does when it speaks of the “one God”.
Some, of course, jump on this as a chance to exclude the Son and Holy Spirit either from existence altogether as the Jews and modalists, or from the divine nature, as the Arians do. Such radical departures from what scripture in totality teaches us are certainly not warranted by this interpretation. For scripture to speak of the Father as the one God, without making explicit mention of the other persons at the same time, does not give us license to ignore what the rest of scripture teaches about God’s only-begotten Son and Holy Spirit. Scripture is clear in teaching that the Son and Spirit are distinct persons from the one God, the Father. Both these persons share the same divine nature as the Father: see: Does teaching the Father is the one God undermine the divinity of Christ? and Why There is Only One God: One Divine Nature.
That Deuteronomy 6:4 is most reasonably taken, then, as speaking of the person of the Father, I have now shown. And on the basis of sound reasoning from the scriptures, this conclusion ought to be accepted. Yet I am aware that many throughout church history have insisted that this verse refers to the entire Trinity, or even to the person of the Son in some stranger interpretations. Because of this, I think it useful to include a few quotes here from the church fathers, showing that several of them also regarded this as an acceptable interpretation:
Ignatius of Antioch:
“For Moses, the faithful servant of God, when he said, “The Lord thy God is one Lord,” and thus proclaimed that there was only one God, did yet forthwith confess also our Lord when he said, “The Lord rained upon Sodom and Gomorrah fire and brimstone from the Lord.”” (Epistle to the Antiochenes, Chapter 2)
“There is then one God and Father, and not two or three; One who is; and there is no
other besides Him, the only true [God]. For “the Lord thy God,” saith [the Scripture], “is
one Lord.” And again, “Hath not one God created us? Have we not all one Father?
And there is also one Son, God the Word…” (Letter to the Philippians, Chapter 2)
Irenaeus of Lyons:
“…or shall it be (what is really the case) the Maker of heaven and earth, whom also the prophets proclaimed,—whom Christ, too, confesses as His Father,— whom also the law announces, saying: “Hear, O Israel; The Lord thy God is one God?”” (Against Heresies, Book 4, Chapter 2)
“For He did not take away the law of nature, but confirmed it. For He that said
in the law, “The Lord thy God is one Lord;”1168 the same says in the Gospel, “That they might know Thee, the only true God.”” (Apostolic Constitutions, Book 6, Section 4, Chapter 23)
“Has then the divine teaching, which abolished the godlessness of the heathen or the
idols, passed over in silence, and left the race of mankind to go entirely unprovided with
the knowledge of God? Not so: rather it anticipates their understanding when it says:
“Hear, O Israel, the Lord thy God is one God;” and again, “Thou shalt love the Lord thy
God with all thy heart and with all thy strength;” and again, “Thou shalt worship the Lord thy God, and Him only shalt thou serve, and shalt cleave to Him.” 2. But that the providence and ordering power of the Word also, over all and toward all, is attested by all inspired Scripture, this passage suffices to confirm our argument, where men who speak of God say: “Thou hast laid the foundation of the earth and it abideth. The day continueth according to Thine ordinance.” And again: “Sing to our God upon the harp, that covereth the heaven with clouds, that prepareth rain for the earth, that bringeth forth grass upon the mountains, and green herb for the service of man, and giveth food to the cattle.” 3. But by whom does He give it, save by Him through Whom all things were made? For the providence over all things belongs naturally to Him by Whom they were made; and who is this save the Word of God, concerning Whom in another psalm182 he says: “By the Word of the Lord were the heavens made, and all the host of them by the Breath of His mouth.”” (Contra the Heathen, Part 3)
“And of the Father it is written, ‘The Lord thy God is One Lord,’ and, ‘The God of gods, the Lord, hath spoken, and hath called the earth;’ and of the Son, ‘The Lord God hath shined upon us,’ and, ‘The God of gods shall be seen in Sion.’ And again of God, Isaiah says, ‘Who is a God like unto Thee, taking away iniquities and passing over unrighteousness?’” (De Synodis, Part 3)