Well, you wouldn't let a guy knocking on your door into your house on the word through the door that he's a cop and has a warrant.. not unless you're an idiot anyway and I don't believe that you are. Instead, you'd do what we all do, ask for ID, a badge, demand to see the warrant and call an attorney before the presumed officer is allowed to step one foot inside to do anything. And that is dealing with your physical self and possession. Yet, you rationalize that when it comes to something far more important, your soul, you should just take their word for it to be nice..
I'll let that sink in.
We differentiate between what is X and what is not X because the only way we know we're not being defrauded is in being sure that X really is X when claiming to be X. Thus, we do not feel it expedient to merely believe that someone or some group is Christian merely because they claim to be and their 'self-esteem' might be damaged by hearing that they are not.
Their self-esteem doesn't trump my right or anyone elses right not to be defrauded.
Look at Revelation wherein followers of the Beast think they are following Christ, yet, by taking the mark of the Beast, they are damned. It rather underscores the need for being sure of what you are following and accepting as true. You can truth someone into heaven and self-esteem them straight to hell.
Lo, the poor Indian! whose untutor'd mind
Sees God in clouds, or hears him in the wind;
His soul proud Science never taught to stray
Far as the solar walk or milky way;
Yet simple nature to his hope has giv'n,
Behind the cloud-topt hill, an humbler Heav'n,
Some safer world in depth of woods embraced,
Some happier island in the wat'ry waste,
Where slaves once more their native land behold,
No fiends torment, no Christians thirst for gold.
To be, contents his natural desire;
He asks no Angel's wing, no Seraph's fire;
But thinks, admitted to that equal sky,
His faithful dog shall bear him company.Alexander Pope, Essay on Man, 1733-1734
However, there is a great difference between making a judgement about whether of not someone is a cop and has a warrant, and whether or not someone is actually a Christian...the steps one would take to verify that the cop is actually doing his required duty, according to the law, and my getting my attorney to my house, are all pretty straightforward things...not really subject to interpretation...
However, when trying to determine if one is a Christian, much of that comes down to ones own personal interpretation of the Bible...and there is the problem...no one on this earth, not one single person, not me, not you, has the market on what the truth of every single passage in the Bible means, or on what every single word means...so much of it comes down to personal interpretation....its not as straightforward as your propose it is...