God did not change, he was changed at the first council of Constantinople in 381
The ambit of your contention is wide. Is the essence of your critique the failure of the church to accomodate the Arians?
If so you will first need to explain what the word "Trinity" means as against the Arian conception of God. Is "Trinity" a supervening non-Arian constitution of God in heaven, as the scholastics conceive of, or is it just an oblique reference to the baptismal formula of Matt 28:19 (known as the Trinitarian formula) without any particular connotations other than what is implied by the formula itself? How was the word "Trinity" understood in this early period of the church? How does Arianism accomodate Matt 28:19?
Then you might trace the "Trinity" back from the first council of Constantinople in AD381, and explain how it differs from the formulations adopted by the Councils of Alexandria in 362, and of Sardica in AD343, and of Nicea in AD325, (and any others), and how these differ from the formulations of the Synods of Antioch convened between AD264 and AD269 in the matter of Paul of Samosata. At what point did Arianism cease to be tolerated?
Did anything really change at t
he first council of Constantinople in AD381? Did these "changes" that you object to happen at previous councils? What were those changes?
Or, is your gripe really against what followed the first council of Constantinople: i.e. the Council of Chalcedon in AD451, & the Second Council of Constantinople in AD553 under Justinian I, who was widely criticized for interfering in the affairs of the church and of making perverse decisions.
NB: a core premise is that the Holy Spirit and the Son and the Father are all capable of being individually blasphemed against, and therefore sinned against, and which likely accounts for the Matt 28:19 "trinitarian" formula in the sense that honor is mandated for the three individually and yet construed as one name (one God):
"Then Peter said, “Ananias, how is it that Satan has so filled your heart that you have lied to the Holy Spirit...
" & Mat 12:32
"It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
"You adulterous people, don't you know that friendship with the world means enmity against God