It is from the “first century believers” that we have the divinely inspired New Testament Scriptures they wrote which teach the Triunity of God both as Jesus taught as well as that revealed by the Holy Spirit. Where else would the later theologians gather the proofs in order to describe/define it? Many, many places in the Old Testament also teach it. Though it may have taken a few centuries for the religious leaders to get around to defining the doctrine in precise theological terms, it was taught and accepted by the Apostles, their disciples and passed down to other Christians. That’s not to say there weren’t controversies and disagreements but there is more than enough in Scripture that God has provided us to prove the truth of the doctrine.
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Actually the first written Gospel was not written until about 40 years after the Death of Christ. First century Christians for the most part had no written Gospels for a number of reasons. While it is true that the Gospels started to be written some 40 years after Christ it took time for them to be copied and shared with other congregations. Most proselyting was done by believers who either heard Christ or His associates or those who heard personally from those same people.
While Mark likely wrote his Gospel in 70AD or so Matthew and Luke likely both wrote their Gospels in approximately 80AD with John writing his in about 95AD.
Mark was a very young man or shall we say teenager when Christ died. While it is possible that he heard Jesus speak we have no record of that. Mark was however a traveling companion of Paul. Paul while not being an associate of Jesus while Jesus was in Mortality did personally meet Him on the road to Damascus. Paul’s writing more than any other Apostle was likely heavily influenced by the Old Testament since he was an educated Jew who was also a Roman Citizen. Paul and Mark traveled extensively and likely heard stories in the congregations they traveled to of the others who had come to those congregations telling what they knew of the Christian stories, the stories of Jesus.
My point being that early Christians did NOT have the New Testament Scriptures to base their theology on, mostly they had oral tradition and occasionally perhaps a partial copy of one of the Gospels or letters of one of the Apostles late in the century. There are many letters however that we don’t have written by associates of the Apostles that were burned at about the same time as the Nicaean Council of 325AD.
During the first two centuries The Church was often in hiding and enduring much persecution so that promulgating copies of the Gospels was next to impossible. Brave souls having acquired a piece or a copy of one of the Gospels would travel from city to city and read what they had to congregations they could get to.
Much of what we have of the first and second century writings are beautiful but theologically some of them are woefully uninformed.
The Canonization of the scriptures in 393 while not the earliest compilation of the New Testament (likely about 350AD) is when intellectual study of the theology of Christianity really took off and came into it’s own. With canonized scripture priests and scribes started copying the “New Testament” in earnest so that many congregations could have part or all of the New Testament.
We don’t now know what was rejected when the books we have now were officially accepted by “The Church” we do know that Canonization of the scriptures stopped much of the confusion and infighting of which doctrine was correct.
The Trinity as we now accept it mostly universally was not anywhere near universal at any time in the first century. As I said earlier, the Council in 325AD made it mandatory but even then it took several more decades for it to be universally accepted.
And my point is that the truth of the Trinity was there all along in the writings of the OT, the teachings of Jesus which were passed down long before there was a "formal" canon and then reinforced by the written revelation given to the ecclesia BY Almighty God and preserved BY Him to this day. God used humans to communicate the truth about His invisible qualities, His eternal power and divine nature - it is even there in creation. What makes anyone think He could be defeated by heretics? Mere humans didn't invent the doctrine of the Trinity.
What is it about the Trinity that you reject? Is it because our finite minds cannot begin to grasp the infinite? I'll give you that, but there are MANY things we believers accept by faith and without faith, it is impossible to please God.
Hope you have a great week.