Understanding theory

I occasionally read something so amazing that I am all but speechless:

 But to argue that you have understood classical theory when you don’t agree with a word of it makes me have to ask why you are so sure you have it right?

So the only people who can understand--and critique--a theory are those who believe--exactly how much of it do I have to believe before I can understand and critique it, again?  And, if the theory is wrong, how much of it do I have to believe before I can understand and critique it?