Correspondingly, I am never surprised when non-Christians insist that they know better who is and who is not a Christian than does a Christian.Gee whiz. I'm never surprised at how many Christians define out of Christianity those Christians who are subjectively the wrong kind of Christians.
What is a Christian? Literally, it is someone who follows Christ. The very word "Christian" was coined as an insult that meant "Little Christ". Therefore, anyone who claims to be a Christian had better be acting like Christ. Do not
Think of it this way, two people claim to be Premier League fanatics. One watches every game he can, reads the teams' statistics and watches player interviews. He is happy when his team wins and sad when it loses. The other only watches American football. He knows the names of two teams in the Premier League and can't name any of the players, much less any of their statistics. It's handy to claim he's a fan because he gets invited to parties.
Which one is a Premier League fanatic? Your standard apparently would say that both are fans because they claim to be.
Two men claim to love the same woman. One spends his time with her, watching her and listening to her, learning what she likes and what she doesn't like. He takes her to dinner. He treats her and her family with respect. The other approaches her and says, "Give me a list of rules to follow so I can claim to love you. I will spend every waking moment trying to get around them and will sometimes flagrantly break them, but I will claim that I love you. In addition, I'll spend as little time with you as possible, and don't expect me to show your family any respect".
Which one loves her? Can you honestly say both do because they both claim to do so?
Or in both of these cases would you not say that the actions of the one demonstrate the truth of his claim while the actions of the other demonstrate the falsity of his claim? Thus it is with Christianity. Religion is dead. Religion insists on rules and regulations. A man in love with his wife doesn't need her to tell him not to fool around with another woman because his love and respect for her tells him not to. A man in love with his wife can gain a lot of wisdom from the advice of his father and grandfather on how to treat his wife, but learns the most by paying attention to what she says, what food she orders at a restaurant, what flowers she likes the most.
See, where your ignorance shows is in you believing that Christianity is a religion. It is not. Religion demands a set of rules, complete with punishment for failure to follow them. Religion is coercion. Christianity is a relationship with Christ. He does not demand obedience to a fixed set of rules, He calls us to love Him and be guided by His Spirit, who tells us what God expects from us and when He expects it. He demands that His followers treat others with love, not coercion.
The bottom line is, you do not understand any of that so you cannot possibly look at someone and say they are a Christian just because they claim to be.