Your question, "The Supreme Spirit?" is answered by me, bu agreeing with you that god is the Supreme Spirit.
Regardless of how the concept / doctrine of Holy trinity was actualised, the facts remain that God the Father, Son, and Holy Spirit are all referred to in the bible. It is this inference that gives rise to the doctrine. It is there, in revealed scripture, not simply formulated at a later date, although I concur that the formulating council simply clarified what was already there.