There are a number of issues here that have been brought up by atheists and skeptics which I find quite legitimate. The first thing they point out is that God is a creation of our own imagination. When Tertullian says "three in form", he has just blundered into a logical fallacy. How can God be a form? A form is a type. It can be subjected to a system of classification, and God cannot be subjected to any system of classification as he is the creator of that system.Tertullian wrote that God is one in substance but three in form. Today we say that God is one in substance but exists in three persons. The change in wording is minor and the concepts are identical.
This leaves me to ask: Is there anything that anyone could have written prior to Nicea which would convince you that they already accepted the doctrine of the trinity?
Other than the person of Christ, the Spirit and the Father cannot be persons. Ultimately Christ transcends his persona, and instructs us to as well, e.g. "Deny youself".
When you speak of identical "concepts" or the "doctrine", it should be pointed out that God is not a concept or doctrine. While this may seem obvious, it shouldn't be overlooked because if we're honest, our only alternative is the transcendent, and the concept of God has no referent. Moreover, existence is not transcendent therefore God cannot exist. What cannot exist, cannot be thought or articulated so whatever one is talking about, it isn't God, but a god of their own imagination; at least insofar as transcendence goes.
I point this out, not to negate what the biblical texts state; but to point out that the reality of Christ in relation to the Father in the power of his holy Spirit is in itself a logical trinitarian confession. It seems to me that as time goes on people fail to grasp the reality and instead simply acknowledge or voice their assent to a doctrine or concept that isn't a part of their reality or awareness.
