"Yeah --- I see what you mean. He for sure was a good source for sound doctrine that was accepted by Christians"
If you had spent just a little more time researching the Montanist movement you would find it had little to do with doctrine but a lot to do with strict discipline at a time when the churches in the larger cities were compromising with the culture. That's why he left the church at Carthage.
I think there most likely was some who tried to be good man to do right thing to the best of their reasoning!
But reasoning is not of the scripture, it is a tool of the world!
I think I saw in his biography where he left the church in a snit because the Bishop of Rome (and the other Christians in Rome) rejected his new doctrines.
That would lead me to believe that his revisionist doctrines that he wanted to introduce were not already accepted by the Christians, and so would be Heresy.
I grant that further into his biography, it says that ultimately the Church Fathers accepted his revisionist doctrines.
That is just like Ptolemy winning out in the end with his theory that the sun revolves around the earth. He may have won out in the end, but it didn't mean that nobody knew what was right, or that he was right. It took clear until the invention of the telescope to straighten that out.
Ultimately his doctrines were accepted by Pope Constantine and the Council he called together. He won the political argument just like Ptolemy and it has lasted just as long. It is also just as wrong.
My original statement was that the early Christians did not believe the doctrine of the trinity. His biography demonstrates that, so under your definition, they weren't Christians either.