They can only be considered a good Christian upto a point because the definition is rather vague and wishy washy and their is a prevalence among other "Christians" not to judge / to be tolerant etc... It's why Christians are far more benign as a whole than Muslims.
Well, a very conservative Christian would say that it is exactly the "poor Christians" who are desperately hanging onto the vishy-vashy part and he follows the real teachings of Paul according to which homosexuality is a sin. Does that mean the more liberal Christians are not "good Christians"? I don't think so.
Now, tell me why ISIS aren't actually the purest and most impeccable followers of Islam.
To cut through all of this: To make any such claim you got to assume that there is a particular way we ought to follow Islam. That Ought must follow from some "is"'s about what is written in books, practices, history, etc. etc.
My argument is going to be that I don't think this is a case where you can go from an ought to an is and someone who is making a claim that Muslims "ought" to be some way is claiming to do exactly that -- something I would like to see him do before I accept it.
If we accept there is no way Muslims ought to behave that can be derived from "is"'s about the Quran, I don't think we can say there is a single way to be a "good Muslim" because "good" is classical "ought"-language.
Alternatively, if you believe there is such a thing as a single way to be a "good Muslim", but that is separate from anything a Muslim ought to do, I think a Muslim (or me) can just reject it because it does not have to do with how a Muslim ought to behave, which is surely what we are concerned about.
Where is the error in the above argument?