Re: You are not a Muslim.
When someone says this to you or about you, why does it bother you so much?
It bothers me so much because Allah says in the Quran that one should not die except in a state when he/she is a muslim. So if someone thinks I am not a muslim, it has to bother me because I do not want to end up dying as a non-muslim. I have to see why he/she thinks I am not a muslim and what I am doing which goes against the teachings of Quran and Sunnah.
Who, other than the Almighty, really has the right to decide whether or not you are?
I guess anyone who has a proper understanding of Islam and its principles can tell whether someone, judging by his/her actions, is a muslim or not.
And if someone decides that you aren't, does that make you non-Muslim?
Depends on what the reasons are for the person to think so. Who knows, unknowingly I might actually be doing something thats not ni accordance with the teachings of the Quran and Sunnah.
Do fatwas issued by ANYONE, really make a difference in your life?
Depends who that ANYONE is.
Allah told the Prophet (saw) in the Quran about some bedouins to inform them that although they say they are believers, they are truly not so, since the faith has not entered their hearts. Now we can't have Allah sending down such verses for ech and every individual so we have to see the scholars' views.