just because a doctrine doesn’t get “formally” defined until later, doesn’t mean it wasn’t something that had already been believed universally by Christians.
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Clearly it wasn’t “already universally believed” by Christians - it was a radical departure: “rife with contention, political agitation, and radical dissension in the early Church.” This begs the question: What did Christians believe BEFORE the bianity 300 years later and the trinity 400 years later?
Not only is there nothing in Tanach supporting Three, everything there supports One and warns of the consequences of deviating from One.
HaShem didn’t make mistakes in the “Old Testament.” There is no need for a “new testament” and in fact we’re explicitly warned about that. The same applies to the Quran, the “final testament.”
One can believe whatever they want and ‘have faith’ in whatever they want but the Tanach is crystal clear on the indivisible, non-partitioned, Unity of HaShem.
How could it be a “radical departure” if the New Testament writings repeatedly affirm it?
As a Christian, I assert that I am a monotheist - that I believe in ONE God. Believing that Jesus the Messiah is the incarnate God and the Holy Spirit is the third person of the Trinity doesn’t mean Christians worship three gods. Jews and Muslims can’t seem to understand that because they refuse to and, because of that, God has allowed a veil of confusion over their eyes. One day, that veil will be removed and the truth will be known by all and at the name of Jesus, every knee shall bow and every tongue will confess that Jesus Christ is Lord to the glory of God the Father.