The early Church Fathers fought against non-Trinitarian concepts that were continuously being promoted as alternatives to traditional teaching.
The apostolic writings of the Bible if read naturally are dead against the concept of Trinity time and again they talk about a son that is subject to the Father rather than being his equal. The early church clearly did not believe in the trinitarian doctrine it had no such tradition.
Not that those who formulated this doctrine had evil motives they were trying to resolve the idea that an inferior (to the father) son was to be worshipped as a God, an absolute no no in monotheism. But if he is the God just as the Father is the God then there is no problem.
The solution is that the Father Himself greatly elevated Jesus's status to that of a God without of course making him his absolute equal. So the early Christians thought of Jesus as a God (but not the God) not surprising since he received all power and authority over man and angel.