Various Answers for the Godhead question
Attempted to preserve monotheism by denying the absolute deity of Jesus Christ. Was a mere man to start. Endued with the Holy Ghost. Eventually Holy Ghost permeated Jesus such that he was deified
Embraced strict monotheism Opposed subordination doctrine (Jesus was lower in the chain of command than the Father). The terms, “Father,” “Son,” and “Spirit” are three titles for the one God as He reveals Himself to mankind relationally and functionally. The Son is the Father manifest in flesh for the purpose of redemption. The Son was only for a period of time. Taught by Noetus, Praxeas, and Sabellius. It was sometimes called Patripassianism (The Father suffered). Close to modern oneness doctrine.
This doctrine teaches since God is immutable, His essence cannot be communicated to any other Therefore Jesus and the Holy Ghost are not God. Jesus was divine but not a deity. Sort of a demi-god point of view.
Teaches that there are three persons within one God. God has one essence but has three persons of that one essence and is coequal, coeternal and consubstantial. Note: true trinitarianism does not teach the subordination doctrine. Most people professing a belief in trinitarianism believe in the doctrine of subordination. The father is unbegotten. The son is begotten.
Additional information on the possible pagan roots of the trinity
The Hindu faith believes this concept of God. The conception most closely linked with Vedism and Brahmanism is that of the Hindu Trinity, the Trimurti. The Absolute manifests himself in three persons: Brahma the Creator, Vishnu the Preserver, and Shiva the Destroyer.
The Egyptian triad of the sun god was “one god expressed in three persons”. He was known as the “noonday sun” (Ra), “the evening sun” (Tum), and “the dawning sun” (Khepera). The sun god reportedly said, “Lo! I am Khepera at dawn, Ra at high noon, and Tum at eventide”. He was one god in three distinct persons.
Father, Son, and Holy Ghost are three separate Gods There is not one essence but three essences Each is similar to each other but separate and distinct (like three men are similar but yet distinct).
Modern doctrine of Oneness
The bible teaches strict monotheism.
Deuteronomy 6:4 Hear, O Israel: The LORD our God is one LORD:
One comes from the Hebrew word אחד (echad) which can mean one (singular) or unity. It appears 885 times in the KJV. Echad means one as in singular 99 percent of the time. Echad is used as unity in such verses as Genesis 2:24. Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Other Scripture talking of strict monotheism
Gal 3:20 Now a mediator is not a mediator of one, but God is one.
Isa 44:6 Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I am the first, and I am the last; and beside me there is no God.
Zec 14:9 And the LORD shall be king over all the earth: in that day shall there be one LORD, and his name one.
The Father is God
Gal 1:3 Grace be to you and peace from God the Father, and from our Lord Jesus Christ,
Mal 2:10 Have we not all one father? hath not one God created us? why do we deal treacherously every man against his brother, by profaning the covenant of our fathers?
Eph 4:6 One God and Father of all, who is above all, and through all, and in you all.
Jesus is God
Col 2:8-9 Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ. For in him dwelleth all the fullness of the Godhead bodily.
Isa 9:6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Mat 1:21-23 And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins. Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
Jesus is the image of the invisible God
Col 1:4 Since we heard of your faith in Christ Jesus, and of the love which ye have to all the saints,
Col 1:15 Who is the image of the invisible God, the firstborn of every creature:
2Co 4:4 In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.
Jesus is God manifest in the flesh
John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God.
John 1:14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
1Timothy 3:16 And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
God declares Jesus is God
Heb 1:1,8 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom.
The Holy Ghost is God
John 4:24 God is a Spirit: and they that worship him must worship him in spirit and in truth.
Eph 4:4 There is one body, and one Spirit, even as ye are called in one hope of your calling;
This is a small sampling of the many scriptures declaring God to be one and manifesting or revealing himself in the man Jesus the Christ.