Christians believe the Father is divine, the Son is divine, and the Holy Spirit is divine. The God of the Bible is the Triune God. Christians worship one God who subsists in three persons. Herman Bavinck described the essentiality of the Triune nature of God when he wrote:
It belongs to God’s very essence to be triune. In that regard, personhood is identical with God’s being itself. “For to God it is not one thing to be and another to be a person, but it is altogether the same thing” (De Trin., VII, 6). For if being belonged to God in an absolute sense and personhood in a relative sense, the three persons could not be one being. Each person, therefore, is identical with the entire being and equal to the other two or all three together.
The three persons of the Trinity are presented in the New Testament as the Father, Son, and Holy Spirit. The three persons of the Trinity share one essence. This singular divine essence is seen in the fact that Jesus taught his disciples to baptize in the “name of the Father and of the Son and of the Holy Spirit” (Matt 28:20).
But when did the Son “become” the Son of God? Did the Son become the Son at some point in Jesus’ incarnate life? Did the Son become the Son at the resurrection? Did the Son become the Son at the incarnation? Or is the Son eternally the Son? Classical Theism teaches that the second member of the Trinity is the eternal Son. Just as the Father has always been the Father and the Spirit has always been the Spirit, the Son is eternally the Son.
Son of God in the OT and NT
We can carry this question back to the Old Testament as well. Psalm 2:7 records the words of the Father to the Son, “The LORD said to me, “You are my Son; today I have begotten you.” In Psalm 2:12, we read that we are to “pay homage to the Son.” The New Testament authors taught that his Psalm is about Jesus (Acts 13:33; Heb 1:5, 5:5). Even in the New Testament, we see these conversations, as it were, between the Father and Son. The Old Testament revealed the Trinity. However, this Trinitarian doctrine was presented as though in darkness and the light was switched on when, by God’s grace, the Son took on flesh (Jn 1) and was proven by the Spirit who recorded the life, doctrine, and church of Christ.
Romans 1:4 says Jesus was “declared to be the Son of God…by his resurrection.” In Matthew 2:15, we see that Jesus was carried down to Egypt so he could be carried out of Egypt, and this would fulfill the words of Hosea 11:1. In Matthew 21:37, Jesus compared himself to the son who was put to death by the corrupt stewards. In Matthew 3:17, the Father said, “this is my beloved Son.” In Luke 9:35, the Father said of Jesus, “this is my Son.” So, when did Jesus become the Son?
Not in Time
So when did the Son become the Son? Never. Jesus is eternally—always—the Son. If we try to pick any point in time for the Son to become the Son, we can only arbitrarily choose a time. The Son never became the Son, just as the Father never became the Father. When the Psalmist wrote “today I have begotten you,” it seems that he did not have a “day” in mind. Instead this was the “day” of eternity. Augustine, in a sermon wrote:
There are two births of our Lord Jesus Christ, the one divine, the other human.… Consider that first begetting: “In the beginning was the Word, and the Word was with God, and the Word was God.” Whose Word? The Father’s own. Which Word? The Son himself. The Father has never been without the Son; and yet the one who has never been without the Son begot the Son. He both begot and yet did not begin to do so. There is no beginning for one begotten without beginning. And yet he is the Son, and yet he is begotten. A mere human is going to say, “How is it that he is begotten, and yet he does not have a beginning? If he does not have a beginning, how was he begotten?” How, I do not know. Are you asking a mere human how God was begotten? I am overwhelmed by your questioning, but I appeal to the prophet: “His begetting who can tell the tale of?
Following Augustine, Turretin wrote, “This generation was made without time (achronōs); not in time, but from eternity.”
This biblical doctrine derived from Scriptural metaphors has been referred to as “eternal generation.” Eternal generation does not teach the inferiority of the Son or that the Son had a beginning or that the Son was brought about by natural means like that of creatures. Instead, the doctrine of eternal generation refers to an eternal personal act of the Father, wherein, by necessity of nature, not by choice of will, he generates the person (not the essence) of the Son, by communicating to him the whole indivisible substance of the Godhead, without division, alienation, or change, so that the Son is the express image of his Father’s person, and eternally continues, not from the Father, but in the Father, and the Father in the Son.—See particularly Heb. 1:3; John 10:38; 14:11; 17:21. The principal Scriptural support of the doctrine of derivation is John 5:26
A. A. Hodge’s definition of eternal generation provides a good introduction to the general thought of the doctrine and the Scriptural support of the doctrine. The titles held by the persons of the Trinity distinguish themselves from the other persons, but these names also teach us about the eternal nature of the persons.
The titles Father and Son are used to describe the Father and Son’s equality and distinguish between them. The Father is eternally the Father, the Son is eternally the Son, and the Spirit is eternally the Spirit. Bavinck wrote, “the divine being is not substantially different from being Father, Son, and Spirit but only relationally.” Turretin also noted how these metaphors or analogies help to present real truths about God’s nature. Turretin noted that the Sonship of the Son teaches that “He is not only of a like (homoiousios), but also of the same essence (homoousios).”
The names Father and Son do not teach that the Son came to be in the same way that human fathers beget their sons. These metaphors do not describe a creaturely begottenness or that the Son is literally a “Word.” Instead, these metaphors help us to understand God better. While the distinction between the metaphor and creaturely reality must be maintained, there are realities communicated to us through the metaphor. Francis Turretin pointed out that “As all generation indicates a communication of essence on the part of the begetter to the begotten (by which the begotten becomes like the begetter and partakes of the same nature with him), so this wonderful generation is rightly expressed as a communication of essence from the Father (by which the Son possesses indivisibly the same essence with him and is made perfectly like him”
The metaphor of “Son” is not the only way in which the nature of the Second member of the Trinity is described. “In Scripture, he bears several names that denote his relation to the Father, such as word, wisdom, logos, son, the firstborn, only-begotten and only son, the image of God, image (εἱκων), substance (ὑποστασις), stamp (χαρακτηρ) [cf. Heb. 1:3].” Again, the metaphors are not to be understood in the same way that they would be literally taken among creatures. Instead, we should remember that God has revealed himself in creaturely terms, which helps us grow in our understanding of the incomprehensible divine nature. The Scriptural metaphors like “Word,” “Son,” and “radiance” are explained with the words “God of God, Light of Light, very God of very God; begotten, not made, being of one substance with the Father.”
There was never a time when the Son was not. The Son is eternally the Word—Jn 1:1. The Son is the Son eternally, and the Father is the Father eternally. These descriptions help us to appreciate the beauty of the Trinity and understand the persons of the Trinity. The eternal Son is the one who was sent to be our Savior (Jn 3:16). The Father, Son, and Spirit are three persons of the same essence. “The distinctness of the individual persons, therefore, arises totally from the so-called “personal properties”: (1) paternity (“unbegottenness,” active generation, and active spiration); (2) filiation or sonship, passive generation, active spiration; (3) procession or passive spiration” The distinctions of the Trinity is distinguished by the distinct relations of Father, Son, and Spirit. The unity of the Trinity is also protected by Father, Son, and Spirit’s eternal relations.
Christians would do well to accept the Biblical metaphors of “Son,” “Word,” and “radiance,” which describe the relation between the Father and the Second Member of the Trinity—the eternal Son. The metaphor of Son teaches Christians that the Son is “generated out of the being of the Father in eternity.” This understanding is in line with the Great Tradition of Christian thought and does justice to the Biblical descriptions of Jesus as “the Son,” “the Word,” and the “Radiance” of the Father. Bavinck summarized the necessity and importance of eternal Sonship when he wrote:
But if the “Father” and the “Son” bear their names in a metaphysical sense, as Scripture incontrovertibly teaches, it follows that the generation in question has to be eternal as well. For if the Son is not eternal, then of course God is not the eternal Father either. In that case he was God before he was Father, and only later—in time—became Father. Hence, rejection of the eternal generation of the Son involves not only a failure to do justice to the deity of the Son, but also to that of the Father. It makes him changeable, robs him of his divine nature, deprives him of the eternity of his fatherhood, and leaves unexplained how God can truly and properly be called “Father” in time if the basis for calling him “Father” is not eternally present in his nature.
 Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 303–304.
 Francis Turretin pointed out that the OT recorded the doctrine of the Trinity in severl passages and that “The principle ones are these three: at the creation of man, “Let us make man in our image” (Gen. 1:26); at the transgression of Adam, “Behold, the man is become as one of us” (Gen 3:22); and at the confusion of tongues, “Let us go down and there confound their language” (Gen 11:7). No reason can be assigned why God (who elsewhere so frequently speaks of himself in the singular) should use the plural verb, unless to intimate a certain (at least) plurality of persons in the unity of essence. Hence he does not say “let me make,” but “let us make,” so that more than one is intimated” (Francis Turretin, Institutes 3.26.4).
 Augustine in “Sermon 196” quoted by Joel C. Elowsky, ed., John 1–10, Ancient Christian Commentary on Scripture (Downers Grove, IL: InterVarsity Press, 2006), 3.
 This eternal Sonship demands that there was no beginning to the generation. However, the concept of “source” remains. Turretin wrote, “Therefore not priority or posteriority of duration can be observed here, although there may be priority of order according to which the Son is from the Father, although not after the Father. (2) Without place (achōristōs) because the Father did not beget out of himself, but in the same essence. Hence the Word (Logos) is said to have been with God, and the Father in the Son, and the Son in the Father. (3) Without any passion (apathōs) or change, either in the Father or in the Son, since that he begat denotes no imperfection, but is rather the reception of all perfection” (Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 1 (Phillipsburg, NJ: P&R Publishing, 1992–1997), 293). This “source” does not imply that the Son is inferior to the Father. Instead, the Father’s creative nature demands that the Son and Spirit proceed from him. Therefore, the Son and Spirit are necessary to the Father’s existence just as the Father is necessary to the Son and Spirit’s existence.
 Archibald Alexander Hodge, Outlines of Theology: Rewritten and Enlarged (New York: Hodder & Stoughton, 1878), 182.
 Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 305.
 Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 1 (Phillipsburg, NJ: P&R Publishing, 1992–1997), 293.
 Irenaeus wrote, “Since therefore His generation is unspeakable, those who strive to set forth generations and productions cannot be in their right mind, inasmuch as they undertake to describe things which are indescribable” (Against Heresies. 2.28.6).
 Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 1 (Phillipsburg, NJ: P&R Publishing, 1992–1997), 292–293.
 Bavinck continued to explain the way the metaphors of begottenness and Word help describe the distinction between the Father and So while also maintaining the unity of the Trinity. He wrote, “Just as the human mind objectivizes itself in speech, so God expresses his entire being in the Logos [Christ]. But here, too, we must note the difference. Humans need many words to express their ideas. These words are sounds and therefore material, sense-related. They have no existence by themselves. But when God speaks, he totally expresses himself in the one person of the Logos, whom he also “granted to have life in himself” (John 5:26 niv).” (Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 308).
 Nicene Creed.
 Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 305.
 Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 309.
 Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 310.