This complex doctrine of one God manifesting as 3 persons came about to allow the belief of Jesus as God without seeming poly-theistic (belief in many gods) since it was the Jewish scriptures that repeatedly said that there was only one God. It also wanted to present Jesus as coeternal with the Father, in contrast to the doctrine of Arius, a priest of Libya, who said "If the Father begat the Son, then he who was begotten had a beginning in existence, and from this it follows there was a time when the Son was not". It was somewhat finalized at the church council of Nicaea in 325 AD. That was basically a council of the leaders of the emerging Catholic church which had been forming since around 100 AD, almost 20 years after Jewish Christians were banned from the synagogues. They didn't want people to think of the Father, Jesus, and the Holy Spirit as 3 different gods, nor of Jesus as being less than God the Father. The doctrine of the Trinity is not taught in the scriptures. It is a man-made doctrine meant to define who the three were and their relationship to each other.
The scriptures do demonstrate unity between God the Father and Jesus (John 10), and between God the Father and the Holy Spirit. But that unity is obviously of mind and purpose, not of singularity (sameness). Why else would the scriptures show each as unique and separate in essence? Why would Jesus pray to the Father if he was the Father? Was he deceiving his disciples? He did say that seeing him was seeing the Father, but he meant that he was showing the attributes of God the Father (love, peace, joy, etc) through his life and therefore was "showing God" even as we Christians "show Jesus".
Jesus said in John 10:30 "I and my Father are one". and further explaining in 10:38 "...the Father is in me, and I in him".
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1 John 5:7 there is a statement that can be misconstrued to promote the idea of
the trinity, but the oneness it states is oneness of mind and purpose, not
“For there are three that
bear record in heaven, the Father, the Word, and the Holy Ghost: and these three
17:11 shows again how the often mentioned “oneness” is unity of mind and
purpose: “Holy Father, keep through your own name those who
you have given me, that they may be one, as we are."
1 Pet 3:8 Finally, be ye all of one mind, having compassion one of another
Rev 17:13 These [ten kings] have one mind
Phil 2:2 Fulfil ye my joy, that ye be likeminded, having the same love, being of one accord, of one mind
Phil 1:27 stand fast in one spirit, with one mind striving together for the faith of the gospel
Eph 5:31 ... a man [shall] leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh
Gal 3:28 for ye are all one in Christ Jesus
2 Cor 13:11 brethren, ... Be perfect, be of good comfort, be of one mind
1 Cor 3:8 Now he that plants and he that waters are one
So far, my studies of the epistles show that Paul and the others would of disagreed with the later church's idea of the Trinity. Take a look at these verses:
1 Titus 2:5 For there is one God, and one mediator between God and men, the man Christ Jesus
1 Cor 8:6 But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him.
Rom 15:6 That ye may with one mind and one mouth glorify God, even the Father of our Lord Jesus Christ.
Click here to read more about the First Church Council of Nicaea