You can't define 'true Christian' because that concept changes according to what is needed by the speaker at the time.
'Not a true Christian' is easier to define. That is anyone who was formerly described as a 'Christian' but is now effing up and/or spouting something hateful or irrational so that other Christians wish to disavow him.
In other forums, when I cite examples of Christian murderers, deviants or hate-mongers, the following people are considered 'not a true Christian':
By all outward accounts, these people seemed to believe as much of the Chrisitan Bible as anyone else, but once you kill 10 million people, then they say you are not a true Christian.