Pick up a set on Ante Nicene Church Fathers if you want to check out what was going on directly after the apostles left the scene. This volume set lists the writings of church leaders until 325 AD. It's easy to read the writings of the leaders that the apostles trained, and who they trained and so forth.
This is the set I bought: https://www.bonanza.com/listings/The-Ante-Nicene-Fathers-Complete-Set-Volumes-1-10-1989-USA-Roberts-Donaldson/1098748708?goog_pla=1
These were the early church leaders and they were always writing to each out about some trouble or heretical group or person and seeking advice from each other. It is very easy to determine what they believed because of all these writings. "Against Heresies" is a major early work. There is some variation among the leaders as you would expect, but nothing of huge importance.
They end up quoting all but a few verses of the NT while writing to each other. So, scriptures that are disputed by the critical text are easily checked out to see if / how the early church fathers quoted them or not. Basically, the early church leaders quoted scriptures consistent with the textus receptus - or KJV.
Lots of groups of Christians were never part of the RCC or the Reformation. Most of their histories were wiped out as the RCC killed 50 million people in Europe over an 800 year period. I do not believe that a person can understand Christian history without learning about these remarkable groups and the methodical relentless persecution they endured for centuries.
Here's the best work documenting these Christian groups IMO: https://www.amazon.com/Reformers-Their-Stepchildren-Dissent-Nonconformity/dp/1579789358
Baptists do not claim to be protestant but claim decendency from these various groups.
