Of course it does.
Telling someone that they are not a Christian is absolutely making a determination about the condition of their soul.
....”Telling someone that they are not a Christian is absolutely making a determination about the condition of their soul”....
Not for someone who might well be unfamilar with what a true Christian is....which in this case could well be.
What you say would be true if “Christian” meant “good person.”
But it doesn’t.