This question is not intended to query Christ's essential deity. All orthodox theologians confess the Christ pre-existed as the eternal Word who was God. But was he eternally the Son of God? We might also include the trinitarian dimensions of our subject. Does the divine name disclosed by Jesus; Father, Son and Holy Spirit (Matthew 28:19), depict God as he is in himself (in se), or simply as he appeared to be for the sake of the economy of redemption (quoad nos)? The New Testament's teaching on Jesus' sonship takes place within the context of the economy of salvation. Scripture does not give us a direct disclosure of the nature of the imminent Trinity. It is as God with us that Jesus is revealed as the Son of the Father. That much is true. But while the imminent Trinity must not be entirely reduced to the economic Trinity, God as he is for us must be a true revelation of God as he is in himself. If God for us is Father, Son and Holy Spirit, it is because he is, ever was, and ever will be Father, Son and Holy Spirit. If this is not the case, we have to ask what, if any correspondence is there between the economic and imminent Trinity?
If we rule out the biblical material that has a bearing on Jesus' eternal sonship because of commitment to preserving the (valid!) distinction between God quoad nos and God in se, then where is our evangelical commitment to sola Scriptura? Surely biblical revelation should weigh more heavily with us than abstract theological reasoning.
What then of the Scriptures? I think that there is a strong biblical case for Jesus eternal sonship.
1) The self-consciousness of Christ
In John's Gospel, the enfleshed Word seemed to be conscious that his Father/Son relationship within the godhead was not simply the product of his incarnate life. Jesus' own sense of his Father/Son relationship with God seems to be elemental to his identity and self-consciousness (John 5:19-29). It is out of love for the Father that Jesus went to the cross (14:31). He referred to himself in relation to the Father in the most deeply personal and intimate way as "Your Son" (John 17:1). He was conscious of his pre-existent glory with God the Father (17:5). The Son hoped to return to the Father, not lay aside a way of relating to God that he assumed only for the purpose of salvation (13:1). Jesus is Son of the Father from eternity to eternity. I think that it would be difficult to argue from the data in John's Gospel that Jesus' sonship was not eternal. What right to we have to bypass the testimony of Jesus' self-consciousness and suggest that in the undisclosed depths of God, he was not in reality the beloved Son of the Father?
2) The mission of the Son
John 3:16 famously teaches that, "God so loved the world that he gave his only begotten Son...". Paul, using similar language tells us that, "In the fullness of time, God sent forth his Son, born of woman, born under the law" (Galatians 4:4). In both texts, it is as the Son that Jesus is sent into the world on a mission of redemption. He did not assume sonship at the incarnation, it was as Son that he was incarnated.
3) The triune name of God
God's key Old Testament name was YHWH - the self existent "I AM", the God of covenant faithfulness (Exodus 3:14, 6:2). This name was revealed in the context of the Old Testament economy of redemption, but who can doubt that YHWH is the revelation of who God is in himself. The New Testament revelation of God's name was announced by the risen Jesus. He commanded that disciples be baptised "in the name of the Father and of the Son and of the Holy Spirit" (Matthew 28:19). This disclosure of God's name is no less a true revelation of God as he is in himself. He is eternally and unchangeably Father, Son and Holy Spirit, just as he is eternally and unchangeably YHWH. If the imminent Trinity is not in fact Father, Son and Holy Spirit, then we must conclude that the economic Trinity is not a fully reliable revelation of God as he is in se.
4) The testimony of Reformed theology
Reformed orthodoxy has almost uniformly insisted that Jesus was the eternal Son of God. Donald Macleod reflects,
"The economic trinity reveals a God if the deepest affection, eternally loving his Son and yet sacrificing his Son for the salvation of a world he made through his Son and which he loves in his Son. The imminent trinity, by contrast, remains undisclosed: a remote reality consisting of God, his Word and his Breath. Without eternal sonship we are left with a redemption which is not a revelation. We have lost the core of the doctrine of the Trinity, namely, that the one Lordship is disclosed as the Father, the Son and the Holy Spirit." (The Person of Christ, IVP, p. 130-131).
John Murray points out the consequences of denying the eternal sonship of Christ,
"If we should deny that the Lord Jesus Christ was eternally the Son of God, then we should have to deny that the Father was eternally Father. For if the first person is eternal Father, it is necessary that there must be a Son of whom he is the eternal Father. And this means that the second person must be eternally Son of the first person. Again, it is in this way that the distinction between the Father and the Son is maintained. It is also very important that, if we deny that the Son was eternally the Son, then we do grave prejudice to the greatness of God's love in sending Christ into the world. The Scripture magnifies the love of God by showing us that it was none other than his own well-beloved and only-begotten Son that the Father sent. He must then have been sent as the Son and not simply to be the Son. It is the greatness of such a gift that advertises the greatness of the Father's love." (Collected Writings Volume 1, Banner of Truth, p. 31-32).
Robert Reymond concludes his consideration of this subject with these words,
"We systematcians should give careful thought to this issue, but I would argue here that we should not discard cavalierly the eternal sonship of the Son in favour of simply affirming the eternal existence of the Logos. For with the rejection of the Son's eternal sonship also goes the Father's eternal fatherhood with tragic results. With what are we then left in regard to distinguishing the properties in the persons of the Godhead that will undergird classical trinitarianism? (Always Reforming, Apollos, p. 103-104.)
Confessing Christ as God's eternal Son in no way makes him subordinate to the Father in the imminent Trinity. As Calvin taught, in regard to his deity, the Son is autotheos, God in his own right, while he is the person of the Son in relation to the Father. We should continue to hold that Jesus was and is the eternal Son of the Father in truth and love.