If you identify with a particular religion or philosophy and label yourself as such, and another comes along with different ideas on what that religion or philosophy is and labels themselves the same as you, does it matter to you? If so, why? If not, why not?
Would you go as far as to tell them that they are a false (insert aforementioned label here)? What can be accomplished by doing so?
Doesn't matter to me at all. Though knowing it will help me in understanding them as a person, at least in part.
As for telling them they are false. Meh imo we (all denominations of Christianity) all have something wrong or misunderstood. I do like discussing the differences in interpretation. Flat out telling someone they are false is useless. Discussing ideas in a civilized manner is the only way to go. Maybe someone changes their mind, maybe not. But the beauty about being Christian is you don't have to be perfect, or else, you wouldn't need to be Christian.