The “Trinity of God”–is it a biblical formulation of God or is it a human formulation that emerged later?
The answer will depend on what you mean by your terms. The word Trinity is not found in the Bible, of course. So, before anyone answers whether the Trinity is of God or of humans, we must first establish the definition of the Trinity. The word was first used, in Latin, by the early church Father Tertullian around 200 AD. The fact that Tertullian invented the Latin word neither proves that it is a biblical concept nor that it is not biblical. We must let the scripture determine whether the word, as used by Tertullian or anyone else is biblical. Tertullian was simply trying to coin a useful word to describe the theology of the nature of God. Do I believe in the Trinity? Is it biblical? Well, first we must have a clear definition. For myself, I find no disagreement with the statements which were produced at the Council of Nicaea and the Council of Chalcedon (see below). Both are in agreement with the biblical view of God to the best that I can tell. Yet, these statements do not use the word Trinity. But they agree with the classical idea of the Trinity, including that of Tertullian. Are we talking about the current Roman Catholic doctrine of the Trinity? Are we talking about some of odd ideas of the Trinity found among Catholic believers in the early Middle Ages (which seemed to imply tritheism)? What Trinity are we talking about?
Let me give you something more concrete to go on. I believe that Jesus is God/deity. This is established beyond a doubt as biblical by dozens of passages. To supply just a few, there is John 1:1, followed by John 1:14. The Word is God and he became flesh. Then there is Colossians 2:9 which tells us that in Christ the fullness of deity dwells in bodily form, and 2 Peter 1:1 and Titus 2:13 which call Jesus our God and Savior. Jesus claims to be God in John 8:58-59 and John 10:30-33. There are so many more. That Jesus is God and that he has all the qualities of deity is biblically established.
I believe that the Holy Spirit is also God/deity. There are a number of passages which clearly establish this. The Father is God, the Son is God, the Holy Spirit is God. They are not merely three different aspects of a single person (this doctrine is known as Modalism and some Pentecostals teach this). These are three entities. Jesus talks to the Father. He tells what the Father has told him to tell us. Jesus goes to the Father so that he can send the Holy Spirit. He (the Holy Spirit) intercedes for us (Romans 8). These are three “persons” (although the analogy to a person may not be absolutely exact, it seems to work). If the doctrine of the Trinity is that Jesus is God, the Holy Spirit is God and the Father is God, yet they are together one God (that there is only one God is established by many dozens of passages) then I guess I do agree with that. If this is the Trinity, then I believe that the idea is from God. Whether I can agree that the Roman Catholic idea of Trinity, with all the baggage it sometimes comes along with, is biblical, is a bit more questionable.
John Oakes (see the Nicene Creed and the Chalcedonian creeds below, compare the to Scripture and see what you think)
We believe in one God,
the Father almighty,
maker of heaven and earth,
of all things visible and invisible.
And in one Lord Jesus Christ,
the only Son of God,
begotten from the Father before all ages,
God from God,
Light from Light,
true God from true God,
begotten, not made;
of the same essence as the Father.
Through him all things were made.
For us and for our salvation
he came down from heaven;
he became incarnate by the Holy Spirit and the virgin Mary,
and was made human.
He was crucified for us under Pontius Pilate;
he suffered and was buried.
The third day he rose again, according to the Scriptures.
He ascended to heaven
and is seated at the right hand of the Father.
He will come again with glory
to judge the living and the dead.
His kingdom will never end.
And we believe in the Holy Spirit,
the Lord, the giver of life.
He proceeds from the Father,
and with the Father and the Son is worshiped and glorified.
He spoke through the prophets.
We believe in one holy catholic and apostolic church.
We affirm one baptism for the forgiveness of sins.
We look forward to the resurrection of the dead,
and to life in the world to come. Amen.
the Chalcedonian Creed:
We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body; consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union,but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.