Why not call Jesus the post-existent Word ?
We could be calling the Word the pre-birth Son.
How can one be a son, before he is born ?
He has always been the Son. It's the doctrine of eternal generation.
Isa 9:6 For to us a child is born, to us
a son is given; and the government shall be upon his shoulder, and his name shall be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace.
Joh 3:16 “For God so loved the world, that
he gave his only Son, that whoever believes in him should not perish but have eternal life.
Joh 3:17 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.
Gal 4:4 But when the fullness of time had come,
God sent forth his Son, born of woman, born under the law,
Col 1:13 He has delivered us from the domain of darkness and transferred us to the kingdom
of his beloved Son,
Col 1:14 in whom we have redemption, the forgiveness of sins.
Col 1:15 He is the image of the invisible God, the firstborn of all creation.
Col 1:16
For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created
through him and for him.
Col 1:17 And
he is before all things, and in him all things hold together.
1Ti 1:15 The saying is trustworthy and deserving of full acceptance, that Christ Jesus
came into the world to save sinners, of whom I am the foremost.
1Ti 3:16 Great indeed, we confess, is the mystery of godliness:
He was manifested in the flesh, vindicated by the Spirit, seen by angels, proclaimed among the nations, believed on in the world, taken up in glory.
Heb 1:2 but in these last days he has spoken to us
by his Son, whom he appointed the heir of all things,
through whom also he created the world.
...
Heb 1:8 But
of the Son he says, “Your throne, O God, is forever and ever, the scepter of uprightness is the scepter of your kingdom.
Heb 1:9 You have loved righteousness and hated wickedness; therefore God, your God, has anointed you with the oil of gladness beyond your companions.”
Heb 1:10 And, “You, Lord, laid the foundation of the earth in the beginning, and the heavens are the work of your hands;
Heb 1:11 they will perish, but you remain; they will all wear out like a garment,
Heb 1:12 like a robe you will roll them up, like a garment they will be changed. But you are the same, and your years will have no end.”
Heb 1:13 And to which of the angels has he ever said, “Sit at my right hand until I make your enemies a footstool for your feet”?
1Jn 3:5 You know that
he appeared [was manifested] in order to take away sins, and in him there is no sin.
...
1Jn 3:8 Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason
the Son of God appeared [was manifested] was to destroy the works of the devil.
1Jn 4:9 In this the love of God was made manifest among us, that
God sent his only Son into the world, so that we might live through him.
1Jn 4:10 In this is love, not that we have loved God but that he loved us and
sent his Son to be the propitiation for our sins.
...
1Jn 4:14 And we have seen and testify that the Father has
sent his Son to be the Savior of the world.
(All ESV.)
While some of the language is ambiguous and
could refer simply to the Son being sent as the man Jesus into the world at the start of his ministry, it
could also mean being sent into the world from heaven, which some of the language is more clear about. Given that John is the only one that uses
Logos of Jesus, it seems clear that he used the word for a specific reason to refer to the Son; the other NT writers only used "the Son" (or Jesus Christ, who is the Son) when speaking of evidence of his eternal preexistence.
If the Son hasn't always been the Son, then that creates problems for the Trinity. It means the Father hasn't always been the Father. It means we can only talk about the Trinity as the First Person, the Word, and the Holy Spirit, or the First Person, the Second Person, and the Third Person. There is no concept of relationship between them, that is, if there is one, we cannot know what it entails.
It also makes it difficult to distinguish between Christ's Sonship and our sonship in which we are adopted. Him being
the Son of God would relate only to his humanity, so in what true sense is he the one and only Son of God? It means he is the Son for all eternity, but that means the Second Person’s relationship with the First Person, if there was one, has changed forever.
Jesus’s Sonship makes the most sense of the text when we understand that it has always been and the Father has always been the Father.