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God the Son (Greek : Θεὸς ὁ υἱός, Latin : Deus Filius) is the second person of the Trinity in Christian theology.  The doctrine of the Trinity identifies Jesus as the incarnation of God, 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).
This section may be too technical for most readers to understand.(November 2022)
The phrase "God the Son" is not found in the Bible,   but is 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: In Greek "God the Son" is ho Theos ho huios (ὁ Θεός ὁ υἱός) as distinct from ho huios nominative tou Theou genitive, ὁ υἱός τοῦ Θεοῦ, "Son of God". In Latin "God the Son" is Deus (nominative) Filius (nominative). The term deus filius is found in the Athanasian Creed: "Et tamen non tres omnipotentes, sed unus omnipotens. Ita Deus Pater, Deus Filius, Deus [et] Spiritus Sanctus." (distinct from filius Dei genitive "son of God"), but this phrase is also translated "So the Father is God: the Son is God: and the Holy Ghost is God". 
The term deus filius is used in the Athanasian Creed and formulas such as Deus Pater, Deus Filius, Deus Spiritus Sanctus: Et non tres Dii, sed unus est Deus. 
The term is used by Saint Augustine in his On the Trinity , for example in discussion of the Son's obedience to God the Father: deo patri deus filius obediens.  and in Sermon 90 on the New Testament "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." 
The Augsburg Confession (1530) adopted the phrase as Gott der Sohn. 
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.' " 
"Son of God" is used to refer to Jesus in the 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". 
The Logos or Word in John 1:1 ("In the beginning was the Word, and the Word was with God, and the Word was God), is often interpreted, especially by Trinitarians, to identify the pre-existent Jesus with this Word.[ citation needed ]
The disputed Comma Johanneum (1 John 5:7) includes the Son in the formula "For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one." 
Christians believe 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] 
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. 
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."
Groups of both trinitarian and antitrinitarian Christians reject the term 'God the Son' to describe Jesus Christ (as well as 'God the Holy Ghost' to describe the Holy Spirit). Jehovah's Witnesses reject the term along with the word 'Trinity' as extrabiblical terminology, along with the Deity of Christ.
Oneness Pentecostals, who affirm his divinity, object to the term as an unauthorized reversal of the language of Scripture which describes him 40 times as the "Son of God."   The Church of Christ, which accepts both the Deity of Christ and the trinity doctrine, also avoids the term because they stress the importance to 'Call Bible things by Bible names, and talk about Bible things in Bible ways.' 
While most mainstream Christian denominations hold God the Son to be "begotten of [...] the substance of" God the Father, and therefore one part of a single whole,  the Church of Jesus Christ of Latter-day Saints holds that God the Father, God the Son, and God the Holy Ghost are in fact three separate beings.  This is not to be confused with the Reorganized Church of Jesus Christ of Latter-day Saints, which does maintain the one-ness of the trinity. 
Arianism is a Christological doctrine first attributed to Arius, a Christian presbyter from Alexandria, Egypt. Arian theology holds that Jesus Christ is the Son of God, who was begotten by God the Father with the difference that the Son of God did not always exist but was begotten/made before "time" by God the Father; therefore, Jesus was not coeternal with God the Father, but at the same time Jesus began to exist outside time as time applies only to the creations of God.
Historically, many rulers have assumed titles such as the son of God, the son of a god or the son of heaven.
In Christianity, Sabellianism is the Western Church equivalent to Patripassianism in the Eastern Church, which are both forms of theological modalism. Condemned as heresy, Modalism is the belief that the Father, Son and Holy Spirit are three different modes of God, as opposed to a Trinitarian view of three distinct persons within the Godhead. However, Von Mosheim, German Lutheran theologian who founded the pragmatic school of church historians, argues that Sabellius "believed the distinction of Father, Son, and Holy Spirit, described in the Scriptures, to be a real distinction, and not a mere appellative or nominal one."
The Christian doctrine of the Trinity is the central doctrine concerning the nature of God in most Christian churches, which defines one God existing in three coequal, coeternal, consubstantial divine persons: God the Father, God the Son and God the Holy Spirit, three distinct persons (hypostases) sharing one essence/substance/nature (homoousion) As the Fourth Lateran Council declared, it is the Father who begets, the Son who is begotten, and the Holy Spirit who proceeds. In this context, one essence/nature defines what God is, while the three persons define who God is. This expresses at once their distinction and their indissoluble unity. Thus, the entire process of creation and grace is viewed as a single shared action of the three divine persons, in which each person manifests the attributes unique to them in the Trinity, thereby proving that everything comes "from the Father," "through the Son," and "in the Holy Spirit."
Nontrinitarianism is a form of Christianity that rejects the mainstream Christian theology of the Trinity—the belief that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being, or essence. Certain religious groups that emerged during the Protestant Reformation have historically been known as antitrinitarian.
Oneness Pentecostalism is a nontrinitarian religious movement within the Protestant Christian family of churches known as Pentecostalism. It derives its distinctive name from its teaching on the Godhead, which is popularly referred to as the Oneness doctrine, a form of Modalistic Monarchianism. This doctrine states that there is one God, a singular divine spirit with no distinction of persons who manifests himself in many ways, including as Father, Son, and Holy Spirit. This stands in sharp contrast to the doctrine of three distinct and eternal persons posited by Trinitarian theology.
God the Father is a title given to God in Christianity. In mainstream trinitarian Christianity, God the Father is regarded as the first person of the Trinity, followed by the second person, God the Son Jesus Christ, and the third person, God the Holy Spirit. Since the second century, Christian creeds included affirmation of belief in "God the Father (Almighty)", primarily in his capacity as "Father and creator of the universe".
The Johannine Comma is an interpolated phrase (comma) in verses 5:7–8 of the First Epistle of John.
Homoousion is a Christian theological term, most notably used in the Nicene Creed for describing Jesus as "same in being" or "same in essence" with God the Father. The same term was later also applied to the Holy Spirit in order to designate him as being "same in essence" with the Father and the Son. Those notions became cornerstones of theology in Nicene Christianity, and also represent one of the most important theological concepts within the Trinitarian doctrinal understanding of God.
In Christian theology, the incarnation is the belief that the pre-existent divine person of Jesus Christ, God the Son, the second person of the Trinity, and the eternally begotten Logos, took upon human nature and "was made flesh" by being conceived in the womb of a woman, the Virgin Mary, also known as the Theotokos. The doctrine of the incarnation then entails that Jesus was at the same time both fully God and fully human—two natures in one person.
Modalistic Monarchianism, also known as Modalism or Oneness Christology, is a Christian theology upholding the oneness of God as well as the divinity of Jesus; as a form of Monarchianism, it stands in contrast with Trinitarianism. Modalistic Monarchianism considers God to be one while working through or existing as the different "modes" or "manifestations" of God the Father, God the Son, and God the Holy Spirit, without limiting his modes or manifestations.
God in Christianity is believed to be the eternal, supreme being who created and preserves all things. Christians believe in a monotheistic conception of God, which is both transcendent and immanent. Christian teachings on the transcendence, immanence, and involvement of God in the world and his love for humanity exclude the belief that God is of the same substance as the created universe but accept that God the Son assumed hypostatically united human nature, thus becoming man in a unique event known as "the Incarnation".
Debate exists as to whether the earliest Church Fathers in Christian history believed in the doctrine of the Trinity – the Christian doctrine that God the Father, God the Son and God the Holy Spirit are three distinct persons sharing one homoousion (essence).
This is a glossary of terms used in Christianity.
Christian theology is the theology of Christian belief and practice. Such study concentrates primarily upon the texts of the Old Testament and of the New Testament, as well as on Christian tradition. Christian theologians use biblical exegesis, rational analysis and argument. Theologians may undertake the study of Christian theology for a variety of reasons, such as in order to:
The Jesus' name doctrine or the Oneness doctrine upholds that baptism is to be performed "in the name of Jesus Christ," rather than using the Trinitarian formula "in the name of the Father, and of the Son, and of the Holy Spirit." It is most commonly associated with Oneness Christology and the movement of Oneness Pentecostalism; however, some Trinitarians also baptise in Jesus' name and interpret it as on the authority of Jesus' name which most of mainstream Christendom justifies as referencing the existence of a Trinitarian Christian deity through the Great Commission among other precepts such as instances in the Old Testament.
For the majority of Christian denominations, the Holy Spirit, or Holy Ghost, is believed to be the third person of the Trinity, a Triune God manifested as God the Father, God the Son, and God the Holy Spirit, each person itself being God. Nontrinitarian Christians, who reject the doctrine of the Trinity, differ significantly from mainstream Christianity in their beliefs about the Holy Spirit. In Christian theology, pneumatology is the study of the Holy Spirit. Due to Christianity's historical relationship with Judaism, theologians often identify the Holy Spirit with the concept of the Ruach Hakodesh in Jewish scripture, on the theory that Jesus was expanding upon these Jewish concepts. Similar names, and ideas, include the Ruach Elohim, Ruach YHWH, and the Ruach Hakodesh. In the New Testament it is identified with the Spirit of Christ, the Spirit of Truth, the Paraclete and the Holy Spirit.
Christian denominations have variations in their teachings regarding the Holy Spirit.
Paterology, or Patriology, in Christian theology, refers to the study of God the Father. Both terms are derived from two Greek words: πατήρ and λογος. As a distinctive theological discipline, within Theology proper, Paterology is closely related to Christology and Pneumatology.
Christian devotional literature is religious writing that Christian individuals read for their personal growth and spiritual formation. Such literature often takes the form of Christian daily devotionals. Original excerpts including the Book of Daniel and Leviticus derive from Ancient Roman, Greek and Byzantine culture – and encompass the past relationship of God's Law through the Old Testament. Though these are the most significant accounts, the majority of the literature comprises commentaries to the ever changing social and political reforms of human history – including the impact of censorship, persecution – the reign of Emperor Nero and Diocletian and martyrdom on Christian life through the ages.
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.