I believe trinities have always existed but that doesn't make them The Trinity.
I believe that is irrelevant.
You are right. Trinities have existed and in pagan roots.
Sumeria
“The universe was divided into three regions each of which became the domain of a god. Anu’s share was the sky. The earth was given to Enlil. Ea became the ruler of the waters. Together they constituted the triad of the Great Gods” ( The Larousse Encyclopedia of Mythology, 1994, pp. 54-55)
Babylonia
“The ancient Babylonians recognised the doctrine of a trinity, or three persons in one god— as appears from a composite god with three heads forming part of their mythology, and the use of the equilateral triangle, also, as an emblem of such trinity in unity” (Thomas Dennis Rock, The Mystical Woman and the Cities of the Nations, 1867, pp. 22-23).
Greece
“In the Fourth Century B.C. Aristotle wrote: ‘All things are three, and thrice is all: and let us use this number in the worship of the gods; for, as the Pythagoreans say, everything and all things are bounded by threes, for the end, the middle and the beginning have this number in everything, and these compose the number of the Trinity’ ” (Arthur Weigall, Paganism in Our Christianity, 1928, pp. 197-198).
“The origin of the conception is entirely pagan”
Egyptologist Arthur Weigall, while himself a Trinitarian, summed up the influence of
ancient beliefs on the adoption of the Trinity doctrine by the Catholic Church in the following excerpt from his previously cited book:
“It must not be forgotten that
Jesus Christ never mentioned such a phenomenon [the Trinity], and nowhere in the New Testament does the word ‘Trinity’ appear. The idea was only adopted by the Church three hundred years after the death of our Lord; and the origin of the conception is
entirely pagan . . .
What is not pagan? From the bible:
1 Corinthians 8:5-6 New International Version (NIV)
For even if there are so-called gods, whether in heaven or on earth (as indeed there are many “gods” and many “lords”),
yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live.