Is the defining quality of Christianity a set of political beliefs based on your personal interpretation of the Bible? Would it be accurate to say, "There's never been a Christian president in the US," if none of them have lived up to your particular moral standards? Do I, and everyone else, have to consult you specifically any time we want to know if someone is or isn't a Christian?

No, obviously not.

Unlike veganism, the question of what the defining quality of a Christian is is more debatable. If you want to define it as, "following Christ's teachings," then it's impossible to establish any sort of reasonably objective standard since people have vastly different interpretations of those teachings. Have you sold all your possessions and given them to the poor? I doubt it. A strict reading of the text might consider that a requirement.

From an academic perspective, it isn't appropriate to weigh in on one's own personal interpretation of which sects and which people should be considered heretical. We should use unbiased terminology that's consistent with common use and can be commonly understood and based on observable things including (but not necessarily limited to) self-identification. When we debate whether or not someone is/was a Christian, trying to match our own personal interpretation of Christ's teachings with our own personal evaluation of their moral qualities would be an absolute nightmare, and it would be impossible to discuss anything past sectarian lines.

And again, it's not just Christianity that this comes up with. A Buddhist might argue that the Japanese temples that endorsed the country's actions during WWII weren't "real" Buddhists, that if they were actually following Buddha's teachings they wouldn't have done that. Should I also consult you personally every time I want to know who is and isn't a Buddhist? Or do I need to read the whole Pali canon and derive my own interpretation and denounce every Buddhist sect that deviates from it as not being real Buddhists - even if I myself am not one and don't have a dog in that fight?