Tertullian certainly wasn't nor were most all of the early church fathers. The Gnostics and disciples of Simon Magus were however followers of Greek philosophy. Irenaeus was a Trinitarian and at the same time wrote against the heresies of these Hellenizing Gnostics.
It does appear that the Ante-Nicene church was split between the trinitarian and the binitarian position, not knowing just exactly if the Holy Spirit was part of the Godhead with the Father and the Son --- but the unitarian position of the Arians led them into some outrageous heretical beliefs.
Your claim that they did not invent the concept of the Trinity is quite valid as it had already existed for over a thousand years.
No --- longer than that. They were the "us" there at creation and from eternity past.
Please explain who exactly the Father, the Son, and the Holy Spirit in the New Testament are.
Just because I don't have a nice, neat answer, doesn't make Rome's answer correct. Working from the standpoint that we actually can figure all this out is itself contrary to scrpture. The notion that we have to know everything assumes that God owes us an explanation and actually puts us in the drivers seat.
1Co 13:9 For we know in part and we prophesy in part;
1Co 13:10 but when the perfect comes, the partial will be done away.
1Co 13:11 When I was a child, I used to speak like a child, think like a child, reason like a child; when I became a man, I did away with childish things.
1Co 13:12 For now we see in a mirror dimly, but then face to face; now I know in part, but then I will know fully just as I also have been fully known.
My biggest theological problem with Jesus being God and man at the same time is that would completely negate His atoning sacrifice. If only His earthly body died, yet his deity lived on, then our sins haven't been atoned for. I believe that God is God and Jesus is His Son, but Jesus has been His Son for eternity and He voluntarily came down here and took human form as part of the plan of salvation laid out at the foundation of the world.
The volume of scriptures where Jesus references His Father in Heaven while He was on the earth is an incredible indication to me that they are two separate entities. They are united and "one" in unity of thought that is not knowable by our carnal, sinful minds. Jesus being seated at the right hand of the Father is also another huge problem for those who believe that Jesus and God are the same entity.
Oh, yeah, the Holy Spirit. I think the Holy Spirit is a power that God doles out to whomever He wishes in order to help htat person make disciples and such. If you look back to 2Kings 13:21, you will see that the Holy Spirit stayed in Elisha's bones post mortem. I don't think that 1/3 of God would hang out in a grave with Elisha just to make a cool scriptural point!