I was raised in a quite studious Trinitarian congregation, and was a regular at bible studies off and on through the youth and adult years, up until the age of 28. I’ve heard the Trinitarian teaching many times.
Don’t you find it odd that the apostles never baptized, repeating the titles, Father , Son, and Holy Ghost? It’s because that they knew those were titles, and not the name (singular).
They knew the name was Jesus. It’s the name that the Son inherited. That’s the name they used in water baptism. The book of Acts bears witness to that fact. It’s the only name under heaven, given among men, whereby we must be saved.
**Further 1 John 5:7 states**
I discussed that in post #82, I you don’t mind checking it out.
I am out of time, but will offer more, hopefully tomorrow night.
I am curious what convinced you they were wrong? Who teaches you now to deny what has been the historic Christian understanding of the triune nature of our God? I certainly don't claim to be able to comprehend everything that this doctrine entails - we have finite minds incapable (for now) of grasping the infinite. This is something we accept by faith because it is what God has revealed to us in His word.
Most still however still subscribe to the 4th century dogmatic creation that compels its adherents to virtually ignore the reports of what was done and completely focus on what was said to prompt them.
The notion that one can rationally twist the most fundamental aspect of the monotheism first propagated by Jews is to my thinking, impossibly irrational. From Deuteronomy 6:4 to the Nicea Council, a bumpy route indeed full of convoluted rationalizations which upon serious reflection defy all aspects of the faith of the children of Abraham.
I am decades beyond trying to figure out why.