then why does the Bible say that he became the Son of God only when he became man?
The Holy Spirit will come upon you, and power of the Most High will overshadow you; THEREFORE also the Holy One who is to be born will be called the Son of God.
For the reason that he became a man he was called the Son of God, the Bible says so. There was no Son of God before that, that is why it is never mentioned in the Old Testament, because the Son of God was and is the Spirit of God who became flesh.
John 1:1-3
In the beginning was the Word, and the Word was with God,
and the Word was God. He was in the beginning with God.
All things came into being by Him, and apart from Him nothing came into being that has come into being.
John 1:14
14 And the Word became flesh, and dwelt among us, and we beheld His glory
These verses in John’s prologue reveal the pre existence of the Eternal Word/Son who was God and became flesh. He was the Creator of all things. Nothing came into existence apart from Him. He is before everything that has a beginning.
John 1:15
15 John bore witness of Him, and cried out, saying, "This was He of whom I said, 'He who comes after me has a higher rank than I,
for He existed before me.'"
John 1:30
"This is He on behalf of whom I said, 'After me comes a Man who has a higher rank than I,
for He existed before me.'
John the Baptist was 6 months older than Jesus Christ. So it is impossible for Christ to be before him unless Jesus pre existed.
John 3:13
No one has ever gone into heaven
except the one who came from heaven — the Son of Man.
Again we see the pre existence of the Son and where He declares that He came from heaven to earth.
John 3:17
"For God did not
send the Son into the world to judge the world, but that the world should be saved through Him.
This verse shows
the Son was sent from heaven by the Father to the earth.
John 6:38
For I have come down from heaven, not to do
my own will but the will of him who sent me.
Human beings come into existence when they are born into this world, but we surely do not come from Heaven.
John 8:23
"You are from beneath
I Am from above, you are of this world I Am not of this world"
John 8:58
Jesus said to them, "Truly, truly, I say to you
, before Abraham was born, I Am."
Here we see that Jesus lets the Pharisees know that He existed as a person before Abraham was born. Once again we see Jesus claiming to be the Eternal God.
Jhn 16:28 I came forth from the Father, and am
come into the world: again,
I leave the world, and go to the Father.
John 17:5
And now, Father,
glorify Me in your own presence with the glory that
I had with you before the world existed.
Here we see
the Son declared that He shared the same Glory together with the Father prior to creation. This passage makes Him equal with the Father as the Eternal God.
John 17:24
"Father, I desire that they also, whom Thou hast given Me, be with Me where I am, in order that they may behold My glory, which Thou hast given Me; for
Thou didst love Me before the foundation of the world.
Below we read it was
the Son who already existed as the Son which the Father sent into the world.
John 3:16-17
"For God so loved the world, that
He gave His only begotten Son, that whoever believes in Him should not perish, but have eternal life. 17 "For God did not
send the Son into the world to judge the world, but that the world should be saved through Him.
1 Cor 15:47 The first man [is] of the earth, earthy: the second man [is]
the Lord from heaven.
Gal 4:4
But when the fulness of the time came,
God sent forth His Son, born of a woman, born under the Law,
1 John 4:14
And we have beheld and bear witness that
the Father has sent the Son to be the Savior of the world.
In
1 John 3:8 we see that
the Son of Gods appearance or manifestation was for this very purpose, that He might destroy the works of the devil. The verb φανερόω means to make manifest, appeared, to make visible or to bring to light something that was previously hidden. This clearly means that Jesus had already existed as the Son of God and He was made manifest or visible to us.
Col 1:13-18
For He delivered us from the domain of darkness, and transferred us to the kingdom of His beloved
Son, 14 in whom we have redemption, the forgiveness of sins. 15 And He is the image of the invisible God, the first-born of all creation. 16 For by Him all things were created, both in the heavens and on earth, visible and invisible, whether thrones or dominions or rulers or authorities — all things have been created by Him and for Him. 17 And He is before all things, and in Him all things hold together.
Heb 1:2-3
in these last days has spoken to us in
His Son, whom He appointed heir of all things, through whom also He made the world. 3 And He is the radiance of His glory and the exact representation of His nature, and upholds all things by the word of His power.
Heb 1:8
But of
the Son He says,
"Thy throne, O God, is forever and ever,
Heb 5:7-8
In the days of His flesh, He offered up both prayers and supplications with loud crying and tears to the One able to save Him from death, and He was heard because of His piety. 8 Although He was a Son, He learned obedience from the things which He suffered.
The question is who or what was He before the days of His flesh? It is obvious that He pre existed before His birth(days of His flesh)
as the Son.
The Father sent the Son into this world and we know this as the Incarnation. Meaning God became flesh( a man). Since the Son is God, the 2nd Person of the Trinity scripture calls the Son- God manifest in the flesh.
hope this helps !!!