You just aren't accepting HIS definition.
Yes, I'm not accepting his definition, but that doesn't mean I have my own definition. Logical fallacy.
You can prove a mathematics problem is incorrect without being able to provide the correct answer.
Mathematics and definitions of words are not the same thing by any stretch of the imagination.
One is way more objective than another and in math, there are established facts. 2 + 2 = 4 no matter where you are in the world. With something as objective as math, yes, you can tell if an answer is wrong even if you can’t determine the right one.
With something subjective like determining whether someone is really a Christian, although Jesus has given us guidelines for making that determination, it is not as easy since no one can know another’s heart.
For something that is subjective, to determine that something is wrong, must mean that you have some idea of what right is, some standard by which to compare your definition to; just like you do for something that is objective. It just isn’t as easy.
So if you’re not accepting his definition, why aren’t you? What criteria are you using for a basis of rejecting his?