I suspect that the concept of a trinitarian deity resulted from 2nd & 3rd century theological debates over the nature & status of Jesus. The Jews, and most likely, Jesus's own disciples were strict monotheists. But after his unexpected & humiliating death, various rumors arose to explain why he didn't fulfill his messianic role of re-establishing the kingdom of Yahweh in Jerusalem. One speculation (based on cherry-picked scriptures) was that he had further work to do on the spiritual plane, so had to return to heaven. But that would require him to be a god himself (or a reincarnation of Elijah), instead of a mere sword-wielding human leader (messiah = royal descendant) of a political rebellion. Some of his recorded statements were sufficiently vague & provocative that various interpretations could apply.
But, for polytheistic gentile converts, monotheism was not inherent in their tradition. So they didn't consider the god-man concept to be blasphemous or sacrilegious (e.g. Pantheon). Also, Jesus had made metaphorical references to the Holy Spirit as-if it was a person, not just a divine force. As the Catholic Church was being cobbled-together from a variety of Jesus cults, their contradictory myths became an obstacle to unification within the Roman empire ("catholic" = universal). So, the leaders from various places began to hold unification meetings in order to hammer-out their differences. Since it was mainly a political argument, they didn't depend on a sign from God, but merely debated & voted, and the majority opinion became the "Truth"
Unfortunately, their good intentions were frustrated, and in order to establish a single authoritative myth & origin-story of the new religion, they were forced to vote the heretics out of their club. But first they had to legally define what beliefs were orthodox, and which were heresy. Ironically, a crux of the debate was on the vexing question of Jesus' role in the religion : god apparition, or inspired prophet, or pretender to the throne, or god-man avatar. Since the latter option was unacceptable to monotheists, the Jewish Christians soon found themselves expelled as reverse Gentiles (literally, not God's people).
Therefore, instead of a continuation of the Abrahamic genetic-tribal heritage of the Jewish religion, Christianity became a distinct new faith-based religion, excluding the Chosen People of the Old Testament God, in favor of the the uncircumcised "Gentiles". With the Jews rejected from their own religious movement, there was no one left, in the radically new Roman imperial Church, to point-out the logical contradictions found in the authorized scriptures of the official state religion. Besides, mystical mythical paradoxes were not unusual in ancient religions. And, the gentiles were typically not as piously legalistic as the Jews. So, even as the theologians continued to debate privately, for the sake of unity, they decided to convince the uneducated common people that a piece of physical (material) bread could also be a metaphysical (spiritual) hunk of flesh. To some, that was a divine spiritual insight. But, the paradox-vs-precept debate continued on the fringes to this day.
