No True Scotsman Fallacy
There is no such thing as "not a true Christian." The only requirement is to self identify oneself as a Christian. It's the reason trasngenderism is a religion; it follows the same principals of declarations of faith.
Now you make think a Christian insincere, but by the doctrine of Christianity, even a true follower of Christ does not know about anyone else's relationship with God. When we die, the only thing the Bible says is man will be judged.