I don't think we need spend any time on the idea that Tertullian introduced something of pagan origin. He states that the teaching is apostolic.
The New Testament makes the following statements:
1. There is only one God, the Father.
2. Jesus is God (worshipped, etc).
3. Jesus is not the Father.
From that, some formulation like the Trinity was inevitable, I suspect.
I would agree with your last statement, but not with Tertullian.
It is a finely weaved web that combined paganism with Christianty. The teaching was never apostolic.
It existed in pagan religions long before Jesus as three gods in one form in India, China, Japan etc...
The New Testament states these three "attributes" of God exist..
1) The Father
2) The Son
3) The Holy Ghost
Now examine each of these attributes.
-The Father is spirit.
-The Son is the only begotten of that Spirit incarnate in flesh. (the fullness of the Godhead bodily)
-The Holy Ghost is spirit.
Jesus said God was His father, and "when you see me you've seen the Father."
The New Testament also says He was concieved of the Holy Ghost.
Unless Jesus had two fathers, the Father and the Holy Ghost are the same Spirit and Jesus the Son was It's only "begotten" human incarnation.
That is one God in human form, not three gods in one form.
It never said they constitute three seperate enities.
The trinity doctrine of "three persons in one God", or "one God with three equal but distinct personalites" and other pagan teachings such as the falsely titled "apostles creed" may have been called apostolic but they were only intoduced and taught by the Christian churches much later after they had began to compromise with paganism