God the Son (Greek: Θεός ὁ υἱός) is the second person, of the Trinity in Christian theology. The doctrine of the Trinity identifies Jesus as the metaphysical embodiment of God the Son, united in essence (consubstantial) but distinct in person with regard to God the Father and God the Holy Spirit (the first and third persons of the Trinity).
In these teachings, God the Son pre-existed before incarnation, is co-eternal with God the Father (and the Holy Spirit), both before Creation and after the End (see Eschatology). Son of God for some draws attention to his humanity, whereas God the Son refers more generally to his divinity, including his pre-incarnate existence.
The term "God the Son" is not found in the Bible, but is a term found in later Christian sources. By scribal error the term is in one medieval manuscript, MS No.1985, where Galatians 2:20 has "Son of God" changed to "God the Son".
The term in English follows Latin usage as found in the Athanasian Creed and other texts of the early church:
The distinction holds true in other modern languages apart from English, for example:
Jacques Forget (1910) in the Catholic Encyclopedia article "Holy Ghost" notes that "Among the apologists, Athenagoras mentions the Holy Ghost along with, and on the same plane as, the Father and the Son. 'Who would not be astonished', says he (A Plea for the Christians 10), 'to hear us called atheists, us who confess God the Father, God the Son and the Holy Ghost, and hold them one in power and distinct in order.' " Saint Augustine in Sermon 90 on the New Testament says, "2. For hold this fast as a firm and settled truth, if you would continue Catholics, that God the Father begot God the Son without time, and made Him of a Virgin in time."
A manuscript variant in John 1:18 (Θεὸν οὐδεὶς ἑώρακεν πώποτε· μονογενὴς Θεὸς ὁ ὢν εἰς τὸν κόλπον τοῦ Πατρὸς, ἐκεῖνος ἐξηγήσατο) has led to translations including "God the One and Only" (NIV, 1984) referring to the Son.
But the term "Son of God" is used to refer to Jesus in the first gospel of Mark at the beginning in verse 1:1 and at its end in chapter 15 verse 39. Max Botner wrote, "Indeed, if Mark 1:1 presents the "normative understanding" of Jesus' identity, then it makes a significant difference what the text includes".
Later theological use of this expression (compare Latin: Deus Filius) reflects what came to be the standard interpretation of New Testament references, understood to imply Jesus' Divinity, but with the distinction of his person from another Person of the Trinity called the Father. As such, the title is associated more with the development of the doctrine of the Trinity. Trinitarians believe that a clear reference to the Trinity occurs in Matthew 28:19, "Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit." John the Evangelist is understood to identify Jesus with the pre-existent Logos or Word, "In the beginning was the Word, and the Word was with God, and the Word was God."[John 1:1], and as the second person of the Trinity, "For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one." [1 John 5:7] 
Christian belief affirms that Jesus is the only begotten Son of God. [John 3:16] Jesus identified Himself in New Testament canonical writings. "Jesus said to them, 'Most assuredly, I say to you, before Abraham was, I AM.' " [John 8:58], which some Trinitarians believe is a reference to Moses in his interaction with preincarnate God in the Old Testament. "And God said to Moses, 'I AM WHO I AM.' And He said, 'Thus you shall say to the children of Israel, "I AM has sent me to you." ' [Exodus 3:14]
There is no such phrase in the Bible, as 'God the Son,' or 'God the Holy Ghost.'
Oneness Pentecostals argue that Scripture never indicates that Jesus' sonship is an eternal sonship. The term 'eternal Son' is never found in the Bible. Nor is the term 'God the Son' in the Bible.
One notes that it does not aspire beyond the pre-trinitarian notion of 'Son of God' to the properly trinitarian idea of 'God the Son.'
... by adding precisely the words that had earlier been omitted, tov viov, but in the wrong place, making the text now read 'faith in God the Son ...' neither of the other expressions ('God even Christ,' 'God the Son') occurs in this way in Paul.