First of all, it is clear that God is described not only as the Sovereign of the universe, but also as having a Son. This is so clear that no commentator, Jew or Christian, Trinitarian or Unitarian, denies this to be the case. Where they disagree is the identity of the "Son" of God.
Second, the Son is clearly a Person and not just a metaphor or an impersonal force or power.
Third, the parallelism in the Hebrew text reveals that what is true of the Father is equally true of His Son. Agur first asks … "What His name [i.e., the Father’s]?" Then he asks … "And what is the name of His Son?"
Notice that Agur asks the same question twice… "What is the name of?…" To lead his reader to the right answer, Agur then issues a rhetorical question… "Surely you know, don’t you?" The obvious answer is, "No, I do not comprehend the nature of the Father or His Son."
The Father and the Son are both described as incomprehensible in their natures because in Hebrew idiom, to know the name of someone is to know their nature. But Agur declares that we cannot know the divine, inscrutable name of God or His Son. Thus, the deity of the Son of God is established in this text. He is just as incomprehensible as His divine Father.
Fourth, the Hebrew parallelism in the text also refutes the attempt to understand the Son as the nation of Israel or one of its earthly kings, which are never said to be incomprehensible. To deny the deity of the Son, in this text, would require one to deny the deity of the Father.
Fifth, Agur could not have uttered these words unless he understood the multi-personal nature of God. R. Payne Smith comments on this verse:
The concluding clauses of this energetic passage are rationally and easily interpreted, if we admit that the ancient Jews had some obscure ideas of plurality in the divine nature.
Keil agrees that this is the underlying assumption of Agur:
But he would not have ventured this question if he had not supposed that God was not a monas [unity] who was without manifoldness in Himself.
(Morey, Trinity: Evidence and Issues [World Bible Publishers, Inc., Iowa Falls, IA 1996], Part II: The Old Testament Evidence, Chapter Eleven, "God the Son," pp. 175-176)
The online NET Bible commentary to verse 4 states:
16sn The reference to "son" in this passage has prompted many suggestions down through the years: It was identified as Israel in the Jewish Midrashim, the Logos or demiurge by some of the philosophers and allegorical writers, as simple poetic parallelism without a separate identity by some critical scholars, and as Jesus by Christian commentators. Parallels with Ugaritic are interesting, because Baal is referred to as a son; but that is bound up within the pantheon where there was a father god. Some of the Jewish commentators exhibit a strange logic in expressing what Christians would say is only their blindness to the full revelation: There is little cogency in this being a reference to Jesus because if there had been such a person at any time in the past he would have left some tradition about it through his descendants (J. H. Greenstone, Proverbs, 317). But Judaism has taught from the earliest times that Messiah was preexistent (especially in view of Micah 5 and Daniel 7); and the claims of Jesus in the Gospels bear this out. It seems best to say that there is a hint here of the nature of the Messiah as Son, a hint that will later be revealed in full through the incarnation. (Source; underline emphasis ours)
The NT documents identify this Holy Son for us, and even give us his name:
"What have you to do with us, Jesus of Nazareth? Have you come to destroy us? I know who you are--the Holy One of God." Mark 1:24
"All things have been handed over to me by my Father, and no one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son chooses to reveal him." Matthew 11:27
"No one has ascended into heaven except he who descended from heaven, the Son of Man. And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life. For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God." John 3:13-18
"For I have come down from heaven, not to do my own will but the will of him who sent me… Then what if you were to see the Son of Man ascending to where he was before?" John 6:38, 62
"The God of Abraham, the God of Isaac, and the God of Jacob, the God of our fathers, glorified his servant Jesus, whom you delivered over and denied in the presence of Pilate, when he had decided to release him. But you denied the Holy and Righteous One, and asked for a murderer to be granted to you, and you killed the Author of life, whom God raised from the dead. To this we are witnesses." Acts 3:13-15
"But the righteousness based on faith says, ‘Do not say in your heart, "Who will ascend into heaven?"’ (that is, to bring Christ down) or ‘"Who will descend into the abyss?"’ (that is, to bring Christ up from the dead)." Romans 10:6-7
"Therefore it says, ‘When he ascended on high he led a host of captives, and he gave gifts to men.’ (In saying, ‘He ascended,’ what does it mean but that he had also descended into the lower parts of the earth? He who descended is the one who also ascended far above all the heavens, that he might fill all things.)" Ephesians 4:7-10
The Son who ascended and descended, the Son who no one knows except God, is the Lord Jesus Christ, God’s Holy One! Thus, Proverbs 30:3-4 is another passage which uses a plural adjective, Qadoshim, because it has two distinct and coequal Divine Persons in view.
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