I can see your point, and I can even agree with you when it comes to trying to designate whether an individual is a Christian or not. However, if you throw your hands up and say “It’s not my place to judge” when it comes to whole groups with well-defined, distinctly non-Christian theology, you’ve lost me.
I mean, say you have children, and one of them comes to you and says they met some nicely dressed young men on bicycles who convinced them that the LDS church is the way to go, would just shrug your shoulders? Wouldn’t you feel it was your responsiblity to council them as to whether, in your opinion, the Mormons were or were not a Christian church, at least at face value?
Absolutely! I try to impart the gospel to my kids and steer them away from weirdness, false doctrines, heresies, paganism, atheism, agnosticism, and similar grave errors and dangers. I don't routinely enter into a really close non-work friendship with someone I believe is in serious error, even if they seem "nice" (though I don't snub them either, because you never know when your own words and actions can be a useful witness for others, nudging them down the right path).
I just mean, let Jesus look into their minds and hearts. He knows every thought, every input, every experience, all the genetic endowments, all the issues that everyone has. I'm not going to go around pointing a finger and screaming, "I have all the answers and you don't, you heathen! You and all your kind are crazy losers!" Because frankly I don't want to wind up with the goats, the ones to whom Jesus says, "Really? Really? You thought you were such a great Christian? A little arrogant, weren't you?"