Your point is well taken.
Definition is everything. If you can convince folks that your definition of a "true Christian" is the one the Christian god likes then the discussion's frame is set. My point about the disappearance of "true Christians" by the first century was meant to highlight the fractionalization that occurred at the very beginning of the movement.
The accusation of heresy by Pflaz is particularly ironic. This was precisely the charge brought by and against the contentious Christian sects in those early days. They continue to tussle to this day.

Jesus was not a Christian. He was a pious Jew.